CLINICAL MICROBIOLOGY REVIEWS, Jan. 2009, p. 13–36
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Vol. 22, No. 1
Acquired Immunity to Malaria
Denise L. Doolan,1* Carlota Doban ˜o,2and J. Kevin Baird3,4
Queensland Institute of Medical Research, The Bancroft Centre, 300 Herston Road, Post Office Royal Brisbane Hospital, Brisbane,
Queensland 4029, Australia1; Barcelona Centre for International Health Research, Hospital Clínic/IDIBAPS, Universitat de
Barcelona, Spain2; Eijkman-Oxford Clinical Research Unit, Jalan Diponegoro No. 69, Jakarta, Indonesia3; and
Centre for Tropical Medicine, Nuffield Department of Clinical Medicine, Oxford University, Oxford, United Kingdom4
LIFE CYCLE AND GEOGRAPHIC DISTRIBUTION............................................................................................14
HISTORICAL OBSERVATIONS OF NAI................................................................................................................15
HISTORICAL OBSERVATIONS OF ACTIVELY ACQUIRED OR INDUCED IMMUNITY...........................16
CHARACTERISTICS OF NAI....................................................................................................................................16
EFFICACY OF NAI......................................................................................................................................................18
EPIDEMIOLOGICAL ASPECTS OF NAI................................................................................................................18
Effect of Exposure.....................................................................................................................................................18
Effect of Age...............................................................................................................................................................20
Insights from Intervention Studies ........................................................................................................................22
ACQUISITION OF NAI...............................................................................................................................................23
Strain-Specific versus Cross-Reactive (Strain-Transcending) Immune Responses ........................................24
Strain-Specific versus Cross-Reactive (Strain-Transcending) Protection...........................................................25
EMPIRICAL OBSERVATIONS OF PREERYTHROCYTIC-STAGE IMMUNITY ............................................27
EMPIRICAL OBSERVATIONS OF ASEXUAL ERYTHROCYTIC-STAGE IMMUNITY.................................27
EMPIRICAL OBSERVATIONS OF TRANSMISSION-BLOCKING IMMUNITY.............................................28
STAGE SPECIFICITY OF NAI..................................................................................................................................28
Each year malaria infects about one-half billion people, kill-
ing 1 million to 2 million and severely dampening economic
development (44, 123, 133, 289, 321a, 321b). The parasitic
Plasmodium species causing malaria persist and even flourish
despite the availability of tools for prevention, control, and
treatment. Those tools consist of an array of drugs, diagnostics,
and insecticides and a detailed understanding of the breeding
site preferences of the many anopheline mosquito vectors.
Despite the tremendous strides in biotechnology during the
past 5 decades and the application to malaria of the many
breakthroughs in molecular biology, genetics, immunology,
and vaccinology by talented researchers, useful vaccines of any
type evade us. This review examines one factor that may con-
tribute substantially to this failure: inadequate understanding
of naturally acquired immunity (NAI).
The dawn of scientific understanding of malaria occurred on
6 November 1880, when Alphonse Laveran observed a male
gametocyte exflagellating in a blood smear from an Algerian
patient with malaria. This event marked the identification of
plasmodia as the cause of malaria (181). Working in India in
1897, Ronald Ross identified plasmodial oocysts in the guts of
mosquitoes fed on parasitemic birds, thereby implicating mos-
quitoes as the vector of malaria (261). William George Mc-
Callum confirmed plasmodial exflagellation as a process of
sexual reproduction in 1897 (200, 201), and Batistta Grassi et
al. confirmed anopheline mosquitoes as the vector of human
malaria in 1900 (130).
Human malaria has persisted through the development of
miracle drugs and insecticides, a global eradication effort, and
30 years of intensive efforts to develop a practical vaccine. Not
only does malaria persist; it thrives. Today the global malaria
situation is “serious and becoming worse” according to the
WHO. The incidence and range of malaria, which were pushed
to lows in about 1965 (the zenith of dichlorodiphenyltrichlo-
roethane spraying campaigns), now increase sharply in areas of
endemicity and spread into areas where control or eradication
had been achieved. Worse still, this resurgence has been in
progress for 40 years. Even as early as 1978 the historian
Gordon Harrison wrote of the persistence of malaria in the
face of such vigorous efforts to attack it, “Failure so universal,
so apparently ineluctable, must be trying to tell us something.
The lesson could be of course that we have proved incompe-
tent warriors. It could also be that we have misconstrued the
problem”(140). Three dominant factors account for the failure
to maintain control: (i) parasite resistance to safe and afford-
able antimalarials, (ii) the almost complete demise of vector
control programs in developing tropical and subtropical coun-
* Corresponding author. Mailing address: Queensland Institute of
Medical Research, The Bancroft Centre, 300 Herston Road, Post
Office Royal Brisbane Hospital, Brisbane, Queensland 4029, Australia.
Phone: (61-7) 3362 0382. Fax: (61-7) 3362 0105. E-mail: Denise
tries, and (iii) the failure to develop a practical vaccine that
prevents malaria. Inadequate understanding of the mecha-
nisms of naturally acquired clinical immunity against plasmo-
dia may be an important factor contributing to the failure to
develop a practical vaccine. We explore this possibility by ex-
amining the genesis and character of the current state of un-
derstanding of NAI.
In 1980, Bruce-Chwatt (50) wrote, “Malaria immunity may
be defined as the state of resistance to the infection brought
about by all those processes which are involved in destroying
the plasmodia or by limiting their multiplication. Natural (in-
nate) immunity to malaria is an inherent property of the host,
a refractory state or an immediate inhibitory response to the
introduction of the parasite, not dependent on any previous
infection with it. Acquired immunity may be either active or
passive. Active (acquired) immunity is an enhancement of the
defense mechanism of the host as a result of a previous en-
counter with the pathogen (or parts thereof). Passive (ac-
quired) immunity is conferred by the prenatal or postnatal
transfer of protective substances from mother to child or by the
injection of such substances.”
In humans, various types of acquired or adaptive immunity
against plasmodia have been defined: (i) antidisease immunity,
conferring protection against clinical disease, which affects the
risk and extent of morbidity associated with a given parasite
density; (ii) antiparasite immunity, conferring protection
against parasitemia, which affects the density of parasites; and
(iii) premunition, providing protection against new infections
by maintaining a low-grade and generally asymptomatic para-
sitemia (171–174, 276). Here, protection is defined as objective
evidence of a lower risk of clinical disease, as indicated by both
the absence of fever (axillary temperature of ?37.5°C) with
parasitemia and lower densities of parasitemia.
Across sub-Saharan Africa where the disease is holoen-
demic, most people are almost continuously infected by P.
falciparum, and the majority of infected adults rarely experi-
ence overt disease. They go about their daily routines of
school, work, and household chores feeling essentially healthy
despite a population of parasites in their blood that would
almost universally prove lethal to a malaria-naive visitor. This
vigor in the face of infection is NAI to falciparum malaria.
Adults have NAI, but infants and young children, at least
occasionally, do not. NAI is compromised in pregnant women,
especially primigravidae, and adults removed from their rou-
tine infections apparently lose NAI, at least temporarily. In-
terventions that reduce exposure below a level capable of
maintaining NAI risk the possibility of catastrophic rebound,
as occurred in the highlands of Madagascar in the 1980s, with
epidemic malaria killing more than 40,000 people (259). Rou-
tine exposure to hyper- to holoendemic malaria protects a
majority of individuals while killing a minority. Aggressive in-
terventions that consider only that vulnerable minority risk
compromising or eliminating the solid protection against se-
vere malaria in the majority.
This review summarizes what is understood about naturally
acquired and experimentally induced immunity against ma-
laria, with the help of evolving insights provided by biotech-
nology, and places these insights in the context of historical,
clinical, and epidemiological observations. Apart from the
practical importance of understanding NAI with respect to
attacking holoendemic malaria, we also undertake this task to
emphasize that NAI may be a good model for vaccine devel-
opment. Consider a vaccine that allows infants and young
children the same immunity enjoyed by their older siblings and
parents: no disease with natural boosting, lifelong. Even if that
concept were to be rendered superfluous by a safe eradication
strategy, a more thorough understanding of NAI would almost
certainly arm vaccinologists with other concepts to explore and
adapt to specific populations. The exploration of NAI is key to
the rational development and deployment of vaccines and
other malaria control tools for almost any population at risk
and, ultimately, a necessary foundation upon which to develop
strategies of eradication by any means.
LIFE CYCLE AND GEOGRAPHIC DISTRIBUTION
Natural transmission of malaria occurs through exposure of
a human host to the bite of an infective female anopheline
mosquito. It is estimated that mosquitoes generally transmit
fewer than 100 sporozoites per bite (241, 260, 306, 307). The
traditional view has been that a feeding mosquito inoculates
sporozoites from its salivary glands into the peripheral circu-
lation of the host, which results, ultimately, in the invasion of
hepatocytes. However, recent studies using intravital imaging
have shown that sporozoites are injected by mosquitoes into
the skin, where they can remain for up to 6 h (323), and that
approximately one-third of those leaving the injection site may
enter lymphatics and drain to the regional lymph nodes (7);
other sporozoites trickle into the bloodstream and traffic to the
liver, resulting in multiple potential sites for sporozoite-host
interaction. In the P. berghei model, up to 80% of sporozoites
injected by mosquito bite were estimated to infect the liver
(122). Despite these relatively high numbers of sporozoites
that leave the skin and move to the liver, the capacity of each
of these sporozoites to result in asexual erythrocytic-stage in-
fections is low. In humans, it takes the bites of five P. falcipa-
rum-infected mosquitoes to ensure that 100% of volunteers
become infected (249, 310). In the P. yoelii rodent malaria
model system, in which one can frequently infect 100% of
inbred mice by intravenous administration of 20 sporozoites
(the 50% infectious dose for BALB/c, C57BL/6, A/J, and
B10.BR mice has been determined to be 4.9 to 10.6 sporozoites
), it takes the bites of six to eight infected mosquitoes to
achieve 100% infection. This indicates that the majority of
sporozoites inoculated by mosquitoes do not lead to produc-
It is now well established that Plasmodium sporozoites mi-
grate through Kupffer cells and several hepatocytes before
finally infecting a hepatocyte (157, 222). In hepatocytes, the
uninucleate sporozoites undergo a cycle of asexual amplifica-
tion called schizogony that lasts 2 to 10 days, depending on the
species (a minimum of 5.5 days among plasmodia infecting
humans), with each exoerythrocytic schizont containing as
many as 30,000 uninucleate merozoites. Clinical symptoms of
malaria are not manifest during liver-stage maturation. In P.
vivax infection, sporozoites can remain within hepatocytes in
dormant stages known as hypnozoites that can cause clinical
relapses (177). Exoerythrocytic schizonts rupture, and the
merozoites are then released into the bloodstream where they
quickly invade red blood cells (90), commencing the erythro-
14DOOLAN ET AL.CLIN. MICROBIOL. REV.
cytic stage of the disease that is responsible for the clinical
symptoms. A recent study with the P. yoelii model shows that
liver merozoites are released from hepatocytes as merosomes,
clusters of 100 to 200 parasites surrounded by a host cell
The invading merozoite carries with it to the interior of the
red blood cell an enveloping membrane derived from the host
cell, and within an associated parasitophorous vacuole the par-
asite commences further asexual development. After matura-
tion from the ring stage (immature trophozoite) to a tropho-
zoite and then to a schizont, the parasite undergoes three to six
mitotic divisions to yield 6 to 36 merozoites within each eryth-
rocytic schizont (ranges depend on the species). After 48 h, the
schizonts rupture, releasing merozoites into the bloodstream.
Some of these invade uninfected red blood cells and repeat the
cycle of blood schizongony.
Other parasites differentiate into male or female gameto-
cytes that circulate independently in the peripheral blood. P.
falciparum gametocytes appear in peripheral circulation about
7 to 15 days after the initial invasion of erythrocytes, but it
remains unclear what factors stimulate gamecytogenesis (105).
Without treatment, most patients with falciparum malaria will
develop gametocytemia within 10 to 40 days after the onset of
parasitemia (80, 82). P. vivax gametocytes, in contrast, appear
in peripheral blood before clinical symptoms. If ingested by a
feeding anopheline mosquito, these forms differentiate to ga-
metes capable of combining to form a diploid zygote where
meiosis occurs. The zygote differentiates to an invasive ooki-
nete that penetrates the gut wall and attaches to the outer
aspect of the mosquito gut. Bathed in hemolymph, the ooki-
nete differentiates to an oocyst which balloons in size as
schizogony produces many thousands of haploid sporozoites.
The mature oocyst ruptures, and the sporozoites migrate
actively to the mosquito salivary glands. The sporozoites
penetrate the glands and rest in the channels bearing saliva,
awaiting access to a vertebrate host.
Four species of plasmodia routinely infect humans: P. fal-
ciparum, P. vivax, P. malariae, and P. ovale. The greatest impact
on human health in terms of mortality is from P. falciparum.
The pan-tropical distribution of the parasite, its potentially
lethal course of infection, and a profile of increasing resistance
to chemoprophylactic and chemotherapeutic agents justifiably
make this species a primary focus of concern. Nonetheless, P.
vivax also constitutes an important burden on public health
throughout most of the tropical and many subtropical or tem-
perate latitudes. Although less often fatal, this infection causes
a severely debilitating disease with frequent and sometime
multiple episodes of relapse. Moreover, recent reports reveal a
significant risk of severe disease and death caused by the same
spectrum of syndromes typically linked to falciparum malaria
(23, 126, 300). Resistance of P. vivax to standard antimalarials
apparently emerged relatively recently compared to that of P.
falciparum (252), but prophylactic and therapeutic failure of
chloroquine against P. vivax now dominates over chloroquine-
sensitive infections on the island of New Guinea (17, 22). At
one study site in Indonesian New Guinea (Papua, formerly
known as Irian Jaya), 95% of patients with slide-proven vivax
malaria had ordinarily curative levels of drug when reporting ill
to clinics (22). Primaquine, the only available drug capable of
preventing relapses of vivax malaria, may routinely fail as a
consequence of parasite resistance, tolerance, or very poor
effectiveness driven by poor adherence to the 14-day treatment
regimen (21). P. malariae also occurs throughout the tropics,
but it tends to appear in isolated pockets and at relatively low
frequency compared to P. falciparum or P. vivax. Chloroquine-
resistant P. malariae has been reported from southern
Sumatra, Indonesia (188). Microscopically confirmed P. ovale
is exceedingly rare in eastern Indonesia, New Guinea, and the
Philippines but is relatively common in West Africa. Little is
known about the clinical susceptibility of this parasite to stan-
dard antimalarials. The geographical distribution, prevalence,
lethality, and drug resistance risk of the human malaria para-
sites are summarized in Table 1.
Recent work by Singh, Cox-Singh, and their colleagues in
Malaysian Borneo provides compelling evidence that a fifth
species may in fact routinely infect humans. They uncovered
evidence of P. knowlesi, which naturally infects macaques on
that island (and many other areas of southeast Asia), routinely
infecting humans living in proximity to the monkeys, causing
acute illness and some deaths (92, 93, 283).
Malaria is not an exclusively tropical disease. Numerous
small outbreaks in North America over the past 20 years em-
phasize the fact that malaria reached well into temperate cli-
mates as recently as 100 years ago. Even today malaria in the
Koreas and much of temperate Middle East illustrates the
ability of these parasites (in particular P. vivax, with its ability
to lay dormant in the liver for weeks, months, or years) to
thrive under seasonally favorable conditions.
HISTORICAL OBSERVATIONS OF NAI
Knowledge of an acquired protection against malaria pre-
dates any information on the specific cause of the disease.
European colonists in the tropics long understood their dan-
gerous susceptibility to malaria compared to indigenous peo-
ple. It was obvious to them that those exposed to malaria since
birth enjoyed a very high degree of protection, although many
attributed such distinctions to genetic constitution (not entirely
TABLE 1. Geographical distribution, prevalence, lethality, and drug resistance risk of the human malaria parasites
SpeciesRange Prevalence Lethality risk
West Africa, Southeast Asia
VOL. 22, 2009ACQUIRED IMMUNITY TO MALARIA 15
erroneously) rather than to acquired immunity. In 1900, Rob-
ert Koch first reported a scientific basis for naturally acquired
protection against malaria (171–174). Using cross-sectional
studies of stained blood films, an innovation at that time, Koch
examined the frequency and density of parasitemia in two
distinct populations: (i) those in an area of low endemicity at
Sukabumi, West Java, and (ii) those in an area of high ende-
micity at Ambarawa, Central Java. Koch deduced that protec-
tion against malaria was acquired only after heavy and unin-
terrupted exposure to the parasite. The relatively uniform
distribution (frequency and density) of parasites across age
groups at Sukabumi contrasted with the distinct age-dependent
patterns seen at Ambarawa. Koch’s report was followed by a
number of cross-sectional microscopic studies of malaria in
communities in Asia and Africa that largely corroborated the
findings on Java. By 1920, the essential features of NAI had
been described. It was accepted that natural immunity was (i)
effective in adults after uninterrupted lifelong heavy exposure,
(ii) lost upon cessation of exposure, (iii) species specific, (iv)
somewhat stage specific, and (v) acquired at a rate which was
dependent upon the degree of exposure (274, 277). These
findings put into context the classic studies of epidemic malaria
in India reported by Christophers (64, 65), Gill (127), Byam
and Archibald (55), and Covell and Baily (88). Epidemic ma-
laria killed tens of thousands of people of all ages because the
level of exposure before the epidemic was not sufficient to
induce protective immunity. Studies of the strain specificity of
NAI awaited therapeutic challenge with malaria for advanced
syphilis in the 2 decades remaining before penicillin appeared
to supplant that therapy.
HISTORICAL OBSERVATIONS OF ACTIVELY
ACQUIRED OR INDUCED IMMUNITY
The advent of “malariatherapy” of patients suffering from
general paresis (neurosyphilis) in 1917 by von Wagner-Jauregg
set the stage for conclusively demonstrating that effective im-
munity against malaria could be induced in humans. Malari-
atherapy effectively cured about one of three individuals
treated in crowded syphilis sanitariums across Europe and the
United States, a feat which earned von Wagner-Jauregg the
Nobel Prize in 1927. A series of published clinical studies
involving thousands of patients largely confirmed conclusions
from field work (summarized in references 89, 161, and 208).
Although achievable after a single infection, induction of ad-
equate protective immunity usually required repeated infec-
tions, and protection against P. falciparum appeared to be
acquired more slowly than that against P. vivax or P. malariae.
Protective immunity to P. vivax did not persist for a long period
of time in the absence of reexposure, as evidenced by the
eradication of acute or chronic vivax malaria in previously
malaria-naive prisoners and subsequent rechallenge studies
with P. vivax sporozoites or trophozoites (326). Immunity was
species specific, since immunity to a particular species con-
ferred no protection against challenge with a heterologous
species (67, 159, 161, 325). However, it was not necessarily
strain specific, since protection could be obtained against chal-
lenge with heterologous strains as assessed by shortened clin-
ical episodes and reduced levels of parasitemia, although the
protection was not quite as effective as that against the homol-
ogous strain (38, 40, 161). Furthermore, it appeared that re-
peated infections brought about a broadening of specificity,
i.e., often transcending the strain (267). A series of retrospec-
tive examinations of the malariatherapy studies with P. vivax
and P. falciparum were published in 1999 and 2004 (80, 82).
The apparently strict species specificity of acquired immunity
described in these treatments figures later in the interpretation
of observations from Indonesian New Guinea where P. falcip-
arum and P. vivax occur together.
After the historic paper by Brown and Brown (47) demon-
strating antigenic variation by P. knowlesi in rhesus macaques,
a theory explaining the slow onset of immunity in the field
emerged. It was reasoned that repeated infections achieved a
sufficiently diverse repertoire of antigenic memory to eventu-
ally defeat most variants encountered in the wild. In other
words, heavy exposure allowed an accumulation of antigenic
memory such that, after 8 to 15 years, there resulted protective
albeit nonsterilizing immunity. The relatively rapid onset of
protective immunity to malaria in neurosyphilis patients re-
ceiving malaria therapy, which was seemingly incompatible
with this interpretation, was turned into supporting evidence
by pointing to the use of homologous strains in many of those
experiments. The often superior protection provided by ho-
mologous versus heterologous strains (which was only obvious
after the first rechallenge) has been the narrow focus of advo-
cates of cumulative acquisition of NAI. Each episode of infec-
tion presumably induced immunity to only that strain. The
broader view shows dramatic strain-transcending protection
that rapidly eradicates the homologous strain advantage with
repeated exposure. Indeed, Brown and Brown (47) themselves
pointed to this phenomenon in an obscure and rarely cited
section of their paper.
An important pitfall in most of those studies, including the
challenge studies using nonhuman primates, was the exclusive
use of adult subjects. The extrapolation of findings for adult
subjects to a human population that includes susceptible in-
fants and children may well have been a critical error. This
issue will be detailed later in this paper.
CHARACTERISTICS OF NAI
The complexity of NAI among age groups increases when
the level of exposure falls below those seen in the Sahel of
Africa, an area of holoendemicity. Across even just Africa,
transmission may be perennial and much less intense, sharply
seasonal and as intense, or limited to sporadic epidemics at
elevations challenging anopheline tolerances and capacity to
serve as a vector. An exploration of the characteristics and
determinants of NAI in these far more complex settings is
beyond the scope of this review. Our purpose here is to explore
the basis of NAI as it occurs in settings of perennial intense
The principal features of NAI have been defined, but little is
known about the underlying mechanisms. The development of
clinical and parasitological immunity to malaria is marked by
the ability to control disease and parasite density. Parasite
density is linked to disease, and diminished parasite counts
almost certainly contribute to diminished risk of disease. The
dominant factor driving protection from disease may be spe-
cific to effectors that diminish parasite numbers, but other
16 DOOLAN ET AL.CLIN. MICROBIOL. REV.
effectors, e.g., responses that diminish proinflammatory cyto-
kines, may also play a role. In areas of heavy transmission, the
prevalence of parasitemia and the risk of morbidity and mor-
tality caused by malaria decrease markedly with age beyond
early childhood. Young children exhibit an “antidisease immu-
nity” which affects the risk and extent of morbidity associated
with a given parasite density. The protection seems to be rap-
idly acquired and results in reduced mortality or severe clinical
disease, at least acutely. In contrast, the seemingly slowly ac-
quired “antiparasite immunity” confers protection against
high-density parasitemia and the attendant risk of severe dis-
ease (192). Sterilizing immunity against infection is never fully
achieved, and an asymptomatic carrier status is the rule among
adults. This phenomenon of a high degree of immune respon-
siveness together with the nearly permanent presence of rela-
tively low densities of parasites was originally described by
Koch in 1900 and is often termed “premunition” (276). In the
absence of continual exposure, the solid immunity against se-
vere disease is apparently relatively short lived. In its usual
context, premunition suggests an immunity mediated directly
by the presence of the parasites themselves and not as much
the result of previous infections. Premunition in helminth in-
fections is a good example; i.e., there is apparent protection
from reinfection with resident worms that is immediately lost
when the worms are eliminated. In the milieu of holoendemic
transmission, it is difficult to sort out which may be the dom-
inant factor of protection seen in older children and adults, i.e.,
circulating parasites or the immunity to them that persists even
in their absence. Passive transfer experiments, described be-
low, demonstrated an acquired and persistent immunity at
work, rather than premunition in its usual context.
In naive individuals of any age, P. falciparum infection is
almost always symptomatic, and clinical symptoms can be ob-
served even at very low parasitemia levels. The immunity to
asexual blood stages where transmission is low or seasonal has
not been adequately explored, and this review does not fully
describe the complexity of those settings. The persistence of
low-grade, asymptomatic parasitemias, as in the Peruvian Am-
azon, for example (42), may involve mechanisms of immune
protection distinct from those occurring in hyper- to holoen-
demic transmission. This review largely excludes immunity in
settings intermediate between exposure of the genuinely ma-
laria-naive and that of the chronically and heavily exposed.
While intermediate exposures and states of immunity clearly
represent important topics, the focus here aims at a grasp of
NAI mechanisms in the perhaps less complex setting of expo-
sure saturation and maximal levels of protection afforded. We
consider this a logical starting point and basis for comparison
for the exploration of immunity in intermediate-transmission
In areas of heavy endemicity, disease is sometimes distinct
from parasitemia, and both are age dependent. Among very
heavily exposed children, high-density parasitemia may occur
in the absence of overt clinical symptoms. The greatest disease
risk for these children is severe anemia rather than cerebral
malaria or failure of respiratory, renal, or hepatic systems.
Adults in such areas rarely have high-density parasitemia, but
when they do, the symptoms appear to be more severe than
those in children with equal parasitemia density and where
there is higher risk of disease (236). These rare symptomatic
adults may represent the small fraction of the adult population
who defy the odds of reinfection long enough for their immu-
nity to wane.
The prevalence of parasitemia increases sharply beginning
at about 20 weeks of age. Nonetheless, children remain re-
markably resistant to high parasitemia, fever, and severe dis-
ease until about 6 months of age. This protection has been
thought to be associated with the presence of maternal immu-
noglobulin G (IgG) antibodies, since IgG is acquired by the
fetus in utero, mainly during the third trimester of pregnancy,
and IgG levels decrease from birth over the first year of life.
However, at least one study has ruled out maternal antibodies
against malaria antigens as the basis of this protection (254).
Alternatively, the protection of infants may be associated with
parasite growth-inhibitory factors such as lactoferrin and se-
cretory IgA found in breast milk and in maternal and infant
sera (165). At between 4 and 10 months of age many infants
are seronegative to specific malaria antigens (1, 254), but the
kinetics of decay of maternal antibodies show interindividual
variations which depend on the antigen and the epidemiolog-
ical setting (119).
The risk of clinical disease increases from birth to about 6
months of age, depending on the transmission rate, and begin-
ning at around 3 to 4 months of age, infants become suscep-
tible to severe disease and death. The risk of cerebral malaria
increases with age in children 2 to 4 years old. At about 2 to 5
years of age, the frequency of clinical disease begins to dimin-
ish and the risk of mortality sharply decreases. The age of onset
of this protection is somewhat earlier with heavier transmis-
sion, but protection rarely occurs before the age of 2 years. The
presence and density of P. falciparum parasitemia at any given
time do not correlate well with clinical disease in areas of
holoendemicity. Children may have high parasite loads but no
symptoms, or they may have disease with low-density para-
sitemia. Parasite density in the peripheral circulation as mea-
sured by microscopic evaluation of blood smears does not
always reflect the full parasite load, as mature parasites may be
sequestered in deep organs. Longitudinal studies measuring
parasitemias in blood daily and autopsy studies quantifying
parasites in tissues are valuable to provide a more complete
picture of the true parasite load. In addition, low-level submi-
croscopic parasitemias may be important in the onset and
maintenance of immunity/premunition.
After the age of peak parasite prevalence, the number of
clinical attacks of malaria per year dramatically declines, as
does the risk of mortality. From adolescence onwards, severe
disease very rarely occurs. Mild clinical episodes may still be
quite common, and the cumulative incidence of parasitemia
often approaches 100% within just a few months (19, 236).
People having chronic, heavy, and largely uninterrupted expo-
sure to infection develop and maintain a highly efficacious
protection from severe disease at an age corresponding
roughly with the onset of puberty. Studies by Kurtis and col-
leagues (178) suggest that the onset of puberty itself, rather
than cumulative exposure linked to calendar age, may be a
dominant factor in the onset of protective immunity. Intrinsic
factors linked to a maturing immune system may be one key to
understanding the molecular and cellular bases of NAI.
The characteristics of NAI against malaria substantially
change during pregnancy. Despite the effective immunity
VOL. 22, 2009 ACQUIRED IMMUNITY TO MALARIA17
against severe disease that comes with reaching adulthood in
areas of heavy endemicity, pregnant women demonstrate a
markedly increased susceptibility to malaria (indicated by in-
creased parasite densities and risk of severe disease and
death), particularly during first and second pregnancies (re-
viewed in reference 212). Significant correlations between host
maternal parity and risk of pathogenic infection have been
reported, and parasite density decreases as the number of
gestations increases (87, 99). Maternal susceptibility to malaria
infection during pregnancy is thought to be related to the
physiological immunosupression that occurs during gestation
(213) and the accumulation of erythrocytes infected with P.
falciparum in the placenta through cytoadherence mechanisms
(28). The specifics of malaria immunity during pregnancy and
the associated pathology have being reviewed recently (29, 124,
256, 257) and will not be considered in detail here; however,
host factors that may in part explain the susceptibility of preg-
nant women to malaria include (i) impairment of cellular im-
munity, as the concentrations of some cytokines (e.g., tumor
necrosis factor alpha) in the placenta have been shown to be
significantly higher among primigravidae with severe anemia
than among other primigravidae and have been correlated with
densities of P. falciparum-infected erythrocytes and of intervil-
lous monocyte infiltrates in the placenta (258), and (ii) hor-
monal immunosuppression, where a sustained increase in the
levels of hormones associated with pregnancy may underlie the
increased susceptibility of pregnant women, particularly primi-
gravidae, to malaria. Levels of corticosteroids, which suppress
cell-mediated immunity, are substantially increased during the
third trimester of pregnancy, in primiparous women, and in P.
falciparum-infected pregnant women compared with other
pregnant women (311). Cortisol concentrations have been
found to be significantly higher in primigravidae than in mul-
tigravidae; conversely, plasma prolactin levels have been high-
est in multigravidae (37). Placental P. falciparum infection is
also likely to have an effect on the development of immunity in
the offspring due to in utero sensitization to parasite antigens
(reviewed in references 45 and 107), and this research area has
recently gained increasing interest.
EFFICACY OF NAI
NAI provides solid protection against severe morbidity and
mortality. Older children and adults in areas of hyper- to ho-
loendemicity rarely experience life-threatening complications
caused by malaria. Even mild disease is relatively uncommon.
Quantitative demonstrations of the relative immunity of adults
living under conditions of hyper- to holoenemic transmission
have corroborated this view. In a study in northern Ghana, an
area of holoendemicity, 2% of adults in a wet-season cohort of
192 individuals had a first parasitemia after radical cure that
was ?20,000 parasites/?l. In that cohort, 97% had a para-
sitemia during the 16 weeks of follow-up (236). In contrast,
32% of infants and young children (age 6 to 24 months) in a
wet-season cohort of 254 had parasite densities of ?20,000/?l.
Most of the subjects with these high-grade parasitemias were
ill, children and adults virtually alike (78 of 81 children with
fever, versus 3 of 4 adults). Prevention of high-density para-
sitemia appears to be the basis of adult protection against
disease. The relative risk of wet-season parasitemia exceeding
20,000/?l in children was 21 (95% confidence interval [CI] ? 8
to 78; P ? 0.00001) relative to adults. In other words the
protective efficacy of NAI against high-density parasitemia was
94%. The efficacy of adult protection against death has not
been similarly measured, but we believe that it must approach
The risk of death among young children should also be put
into the perspective that includes risk of the same among those
who lack continuous chronic exposure to infection. There is no
precise estimate of the risk of death with P. falciparum in the
absence of immunity or chemotherapy in any age group. How-
ever, fatality rates among nonimmune Europeans traveling in
areas of holoendemicity in Africa during the 19th century pro-
vide some clue to the magnitude of risk in malaria-naive adults.
McGregor (207) recounted a European expedition on the Ni-
ger River during the middle of the 19th century in which 28%
of Caucasians died of fevers presumed to be malaria, despite
the likely availability of quinine. Mortality due to malaria
among French troops posted in Senegal between 1819 and
1831 was reported to range from 9% to 57% (95). One may
thus conservatively estimate 30% as the risk of death without
acquired immunity or adequate chemotherapeutic manage-
ment of malaria. In contrast, an estimated 2% of African
children succumb to death caused by malaria before the age of
5 years (131). Thus, it seems that the African children, even
those within the ages of highest vulnerability, enjoy quite a
large degree of protection from severe disease and death. A
study by Gupta et al. suggested that this is the case in areas of
intense transmission, showing by mathematical modeling that
immunity to noncerebral severe malaria may be acquired after
only one or two infections (133a). The basis of that protection
and how it fails in 1 million or so African children each year are
very poorly understood. One study pointed to gamma inter-
feron responses as the key difference between age-matched
African children suffering mild or severe malaria (183). Like-
wise, it is not understood how changes in exposure to infection
would affect that protection in either children or adults.
The early susceptibility of children, the seemingly long pe-
riod of exposure for onset of protective immunity arising from
its strain specificity, the requirement for nearly continuous
exposure, and its nonsterilizing activity among the most pro-
tected have all discouraged vaccine strategists. We offer a dif-
ferent perspective in this review, i.e., that strain-transcending
NAI may be acquired relatively quickly, after as few as three or
four exposures, providing highly efficacious protection from
hyperparasitemia and the attendant risk of severe disease. The
key to understanding this perspective lies in careful consider-
ation of the epidemiology of NAI in distinct settings of expo-
sure. Such understanding reveals key insights of immediate
relevance to vaccine development strategies and technical ob-
EPIDEMIOLOGICAL ASPECTS OF NAI
Effect of Exposure
The distribution of malaria morbidity and mortality within
communities depends directly upon transmission intensity.
Traditionally, the level of malaria endemicity has been classi-
fied by reference to the spleen rate (the proportion of children
18DOOLAN ET AL.CLIN. MICROBIOL. REV.
with an enlarged spleen in a sample of the population) in
malarious areas (50). More recently, however, it has been
recognized that the entomological inoculation rate (EIR),
which is the number of infectious mosquito bites received per
person per unit of time (185), is a more direct measure of
transmission intensity (30). In situations with annual EIRs of
below about 10, the malaria prevalence rate is almost directly
proportional to the EIR, and malaria transmission tends to be
unstable and is considered to be of low to moderate intensity.
At annual EIRs of above 10, individuals receive multiple in-
fectious bites, and malaria transmission intensity is considered
high and tends to be stable (321a). Where the risk of infection
is low, almost all exposed people are at a substantial risk of
debilitating or severe disease. Where the risk of infection is
high, the risk of severe disease is limited to visitors, infants,
young children, and pregnant women. In general, the more
intense the transmission, the earlier and more confined the age
range of susceptibility to disease. Significant associations have
been shown between the intensity of exposure to biting infec-
tious mosquitoes (EIR) and the incidence and density, but not
prevalence, of P. falciparum parasitemia in children 6 months
to 6 years old who reside in areas of endemicity (27, 31, 202,
203, 301, 304). The incidence of primary clinical episodes
among the susceptible subpopulations usually peaks during the
high-transmission season and decreases considerably during
the low-transmission season.
Among people from areas where the disease is not endemic,
a clear correlation exists between the severity of clinical dis-
ease and the density of P. falciparum asexual parasitemia (116,
221, 302, 303). High-density parasitemia constitutes a signifi-
cant risk factor for a poor clinical outcome. In areas of ho-
loendemicity, the prevalence of hyperparasitemia correlates
with an exaggerated risk of cerebral malaria, severe anemia,
hypoglycemia, lactic acidosis, and respiratory distress (321), all
linked to high risk of death (189). However, that correlation
often fails among individuals as opposed to populations. Par-
asitemia among patients with severe malarial anemia tends to
be low compared to that among less anemic patients. None-
theless, prior bouts of hyperparasitemia in such patients very
likely account for the severe anemia despite low parasitemia
concurrent with anemia (210). In areas of endemicity, the rate
of exposure to infected mosquitoes (EIR) has been correlated
with the density but not the prevalence of parasitemia (202),
although as stated above, the density of parasites in the pe-
ripheral circulation does not account for those parasites that
may be sequestered. Nonetheless, it has been proposed that
parasite density may be used as a surrogate marker for mor-
bidity and mortality associated with malaria (27, 202). It fol-
lows from the correlation between density of parasitemia and
the EIR (202) that the degree of exposure to biting infectious
mosquitoes may determine the risk of death. This is a vitally
important point that may help define technical objectives in
malaria intervention strategies in areas of holoendemicity, in-
cluding vaccination. If decreasing parasite loads in the human
host, by any means, creates a corresponding decrease in the
risk of death, striving to do so merits attention.
Several sets of experimental data support the hypothesis (27,
202, 203) that interventions that reduce P. falciparum trans-
mission intensity will reduce high-density parasitemia and ma-
laria-associated morbidity and mortality. Insecticide-treated
bed nets (ITNs) provide protection against morbidity and mor-
tality attributable to malaria (4, 34, 239, 299). This is also true
with regard to the risk of fever per se (5, 26, 278) and the
prevalence or incidence of parasitemia (26, 97, 278, 291).
Three proposed explanations for the association between
intensity of transmission and parasite density seem likely (27).
First, decreased parasitemia as measured in low-transmission
seasons may be due to acquired immunity, since such children
would have recently experienced high-transmission exposure.
Second, a higher intensity of exposure to sporozoites may in-
crease the likelihood of being exposed to a strain of parasite to
which one has not yet acquired protective immunity. Finally,
increased exposure to infected mosquitoes may result in more
sporozoites reaching the liver and consequently more parasites
developing to mature liver-stage schizonts, which then rupture,
resulting in more infected erythrocytes. An inverse correlation
between the number of infectious bites and the prepatent pe-
riod has been reported in experimental challenge studies
In The Gambia and Kenya, the risks of severe malaria (ce-
rebral malaria or severe malaria anemia) in childhood were
reported to be lowest among populations with the highest
transmission intensities, and the highest disease risks were
observed among populations exposed to low to moderate in-
tensities of transmission (195, 196, 202, 203, 292). Snow et al.
(292) argued that interventions that diminish the risk of infec-
tion may actually increase the risk of poor clinical outcomes.
That interpretation provoked some controversy (96, 220). The
difficulty was in grappling with an apparent paradox: a greater
risk of infection yields a lower risk of disease, and attacking the
risk of infection yields a lower risk of disease. We assert that no
paradox exists and that the viewpoints are reconcilable when
put in the context of quantitative risk of infection and disease
and of NAI. The thresholds of exposure leading to clinical
immunity or to a high risk of severe disease are not necessarily
superimposed. An intervention that pushes the attack rate
below the threshold of risk of severe disease does not neces-
sarily cross below the threshold of exposure needed to sustain
acquired clinical immunity. However, there must be a thresh-
old of exposure to sustain clinical immunity, and it can be
crossed by interventions that diminish the risk of infection. If
this threshold is crossed, an increase in susceptibility to less
frequent episodes of infection may occur. This may be visual-
ized by the hypothetical relationship illustrated in Fig. 1. The
obvious solution is recognizing these thresholds and applying
interventions appropriately. However, as discussed below, age
is a critical factor that compounds the complexity of such
The results of a recent modeling exercise to develop an
age-structured mathematical model of malaria transmission to
investigate the processes driving NAI (117) support this
threshold hypothesis. The authors of that study concluded that
the epidemiological age-prevalence curves seen empirically are
best reproduced by a model comprising a form of clinical
immunity that reduces susceptibility to clinical disease, devel-
ops with age and exposure, and is relatively short lived (half-
life of ?5 years) as well as a form of antiparasite immunity that
reduces parasitemia, is acquired later in life, and is relatively
long lived (half-life of ?20 years). Other data on the effect of
intermittent preventive treatment (IPT) and ITNs on rebound
VOL. 22, 2009 ACQUIRED IMMUNITY TO MALARIA19
of malaria, or the lack thereof, are also consistent with this
hypothesis; specifically, it has been proposed (132, 214, 295)
that the extended period of protection observed in some (269,
270) but not all (8, 132, 214) studies following cessation of
intervention may be due to a situation (such as a partially
effective drug) which allows for low-level and persistent para-
sitemia and, consequently, prolonged stimulation of the im-
mune system. It is also possible that the extended protective
effect noted with the RTS,S vaccine in the field (3, 6, 309) may
be due to the induction of blood-stage immunity in some vac-
cinees as a result of leaky RTS,S-elicited protection (295, 319).
An alternative explanation to an extended protective efficacy
following cessation of intervention (IPT, ITN, or vaccine) may
be a decrease in the malaria transmission rate during the study
period as a result of a very successful intervention (129). A
recent meta-analysis of hospital data from areas with differing
transmission intensities in Africa found that the mean age of
individuals experiencing clinical attacks of malaria increased
with decreasing transmission intensity but that the total num-
ber of clinical episodes was similar until transmission dropped
below a certain threshold (233).
Effect of Age
The risk of disease hinges upon both the age of the host and
the intensity of exposure to the parasite. Certain manifesta-
tions of clinical malaria (e.g., anemia) become less severe with
age, and others (e.g., cerebral malaria) become more severe
with age (193). Age is a significant risk factor for the preva-
lence and density, but not the incidence, of parasitemia (27,
203). The main determinant of the age distribution of morbid-
ity is the development of antiparasite immunity that restricts
the density of asexual parasitemia. In areas of high endemicity,
the prevalence and density of P. falciparum parasitemia and the
incidence of overall fevers and of malaria-associated fevers in-
crease with age for the first 6 months of life and then gradually
decline. Parasitemia peaks in children less than 5 years old and
subsequently declines in an age-dependent manner. A signifi-
cant association between age and parasite density among chil-
dren between 6 months and 6 years has been demonstrated,
with younger children more likely to have a density of ?5,000
parasites/?l (27, 202). This association is even more pro-
nounced with parasite densities of ?20,000/?l. Where the at-
tack rate is lower, peak prevalence occurs in older individuals.
Paradoxically, a pathological response is triggered at a lower
parasite density threshold among older individuals, and the
absolute risk of clinical disease at a given parasite density is
higher in older individuals (237, 272, 288).
In populations living permanently under heavy exposure to
infection, separating the possibly independent effects of age
and of cumulative exposure is very difficult. The key determi-
nant of the conspicuous differences in their relative suscepti-
bilities would appear to be their cumulative exposure to infec-
tion. Protection among adults may be presumed to be the
cumulative product of many dozens of infections, and the sus-
ceptibility of children would be due to their relative paucity of
experience with infection, accumulating at a rate of about 5 to
10 per year. However, what if only five infections within a
single year were sufficient for onset of solid clinical immunity,
as occurred in the neurosyphilis patients? The relative failure
of children to develop immunity would likely be the product of
innate differences in how their acquired immune systems func-
tion compared to those of adults. A population having abrupt
and continuous exposure to infection is required to see such
Cross-sectional studies of malaria-naive transmigrants in In-
donesian Papua suggest that intrinsic features of the immune
system that change with age may be key determinants of NAI
against falciparum malaria (12). A relatively rapid acquisition
of protective immunity against parasitemia and mild disease
caused by P. falciparum, but not P. vivax, was observed in
Indonesia (15, 20). The data shown in Fig. 2 illustrate those
findings. In malaria-naive individuals abruptly and perma-
nently exposed to the hyper- to holoendemic malaria of Papua
(formerly known as Irian Jaya), the age-specific prevalence of
parasitemia was at first uniform among age groups, but after 18
to 24 months a distinct age-dependent pattern emerged. This
pattern paralleled that seen in lifelong residents of the area.
Acquired immunity in the transmigrants was apparently not
the cumulative product of many years of heavy exposure but
was the product of recent exposure and intrinsic characteristics
of the acquired immune response that change with age.
More recently, a longitudinal cohort of 243 transmigrants
was followed from the day of their arrival in Papua in August
1996 until July 1999 (176). The key findings from that study
include the demonstration of quantitatively equal risks of in-
fection for children and adults (24) and the identification of
four infections within any 24-month period as the threshold for
onset of clinical immunity (16). Protection from fever occurred
FIG. 1. Hypothetical immunity-exposure curve, showing the hy-
pothesized rise and fall of host susceptibility to severe disease with
falciparum malaria. Segment A shows an increasing risk of death,
principally from hyperparasitemia and cerebral malaria (and perhaps
from respiratory and renal failure), rising with an increasing risk of
exposure to infection. Segment B shows a declining risk of death with
the onset of sufficient exposure to induce NAI. Segment C represents
the threshold of exposure that maintains maximum NAI. Segment D
shows an intensity of exposure that overwhelms NAI and where the
risk of disease states such as severe anemia becomes predominant.
(Reproduced from reference 13 with permission of the publisher.)
20 DOOLAN ET AL.CLIN. MICROBIOL. REV.
only among subjects having low-density parasitemia. This lon-
gitudinal study corroborated the results and assumptions from
the earlier cross-sectional studies in the same region. In all of
the studies in Indonesian New Guinea, coinfection with P.
vivax routinely occurred, and confounding of the onset of NAI
by that infection cannot be ruled out. Nonetheless, studies of
the neurosyphilis patients demonstrated that prior immunity
against one species offered no advantage in terms of onset of
immunity against the subsequent species. Moreover, Barcus et
al. (24) demonstrated that among the Indonesian migrants,
prior infection with either species did not mitigate the risk or
clinical course of immediately subsequent infection by the
Another finding in the studies of Javanese transmigrants in
Papua highlights the importance of innate age-dependent (and
cumulative exposure-independent) differences between im-
mune responses to acute versus chronic exposure to infection.
Within 3 months of arriving in Papua, an epidemic of falcipa-
rum malaria occurred among transmigrants (15). In this group
up to 74% were infected, and nearly all residents reported
clinical symptoms of malaria. During this period, the number
of emergency medical evacuations sharply increased among
adults but not children (18). During the peak month of the
epidemic, 48 adults were evacuated, compared to just 7 chil-
dren (comprising 60% and 40% of the population of that
month, respectively). These numbers provided incidence den-
sity of emergency evacuation values of 1.3 events per person
year among adults and 0.23 event per person year among
children (relative risk ? 2.7; P ? 0.0001; 95% confidence
interval ? 1.9 to 3.8). The incidence of evacuation among
adults fell as sharply as it rose and was indistinguishable from
the rate among children for the next 2 years. Another trans-
migration village in the same region has been similarly retro-
spectively analyzed, with essentially the same findings: adults
were initially at a greatly exaggerated risk of severe disease
compared to equally exposed children (14). The longitudinal
cohort in Papua did not yield information on the risk of severe
disease because the close follow-up and prompt therapy virtu-
ally precluded the possibility of severe disease outcomes. How-
ever, among the eight cases of malaria severe enough to
prompt on-site intravenous quinine therapy, only one was in a
A review of the literature regarding epidemic malaria has
corroborated the susceptibility of adults to severe disease
caused by P. falciparum that was observed in the transmigrants.
Table 2 summarizes the absolute or relative rates of death or
severe disease among children and adults under conditions of
acute exposure to P. falciparum (i.e., epidemic or travel). In all
instances of epidemic malaria where epidemiologic data re-
vealed age-specific rates of severe morbidity or mortality, the
data consistently showed higher rates among adults than chil-
dren. The data illustrated in Fig. 3 contrast patterns of age-
dependent immunity under conditions of chronic versus acute
exposure. A recent report by Dondorp et al. (103) also corrob-
orated the conclusions drawn from the studies of Indonesian
migrants; i.e., hospitalized nonimmune adults were at a higher
risk of severe disease and death caused by falciparum malaria.
All of these findings underscore the apparently dominant effect
on clinical course of disease exerted by intrinsic age-related
distinctions, about which we know very little in the context of
Thus, age-dependent immunity occurs in both acute and
FIG. 2. Onset of age-dependent NAI. The graph illustrates the
prevalence of parasitemia (percent) across age groups (years) among
malaria-naive newcomers to Indonesian New Guinea. After 8 months
of exposure, all ages appear to be equally susceptible to parasitemia
detectable by microscopic diagnosis. One year later (20 months), a
distinct age-dependent pattern of susceptibility to parasitemia has ap-
peared. These data revealed the onset of NAI to be dependent upon
intrinsic age-related factors independent of lifelong, chronic exposure.
(Reproduced from reference 11 with permission of the Liverpool
School of Tropical Medicine.)
TABLE 2. Age-dependent risk of death with falciparum malaria
Old vs young cutoff
Odds ratio (95% CI)Reference
Hospital ICU patients
Hospital ICU patients
VOL. 22, 2009 ACQUIRED IMMUNITY TO MALARIA21
chronic exposure to infection, with apparently critical effects
upon the course of infection that differ sharply between chil-
dren and adults: adults apparently are better able to withstand
chronic exposure, whereas children fare better with acute ex-
posure. Under conditions of chronic exposure, adults appar-
ently develop antiparasite immunity and clinical immunity
more rapidly and completely than children, such that after
about 18 months they have fewer and lower-density para-
sitemias with reduced clinical symptoms. However, under con-
ditions of acute exposure, there is an inverse pattern of relative
susceptibility, i.e., a higher risk of severe disease among adults
than among children. Figure 4 illustrates a possible explana-
tion for this inversion of susceptibility/resistance. The immune
mechanisms underlying this inversion are unclear, but data in
a rodent model may shed some light. In one study, adult rats
were shown to have a higher degree of sensitivity to lipopoly-
saccharide than young rats (68), and this feature was correlated
to sensitivity to infection by P. berghei. Thus, a relatively exag-
gerated production of tumor necrosis factor alpha by adult
humans in response to a primary infection by P. falciparum that
wanes with continued exposure could explain these differences.
Insights from Intervention Studies
The use of intermittent treatment or prophylaxis to control
morbidity and mortality among youngsters in sub-Saharan Af-
rica represents another good example of the importance of
understanding NAI and the consequences of interventions that
affect exposure. Some have argued that interventions such as
intermittent treatment or prophylaxis would set up children for
more severe (cerebral) malaria when the intervention was ul-
timately withdrawn (182, 290). Intervention could perhaps in-
terrupt the accumulation of sufficient antigenic diversity on the
way to children becoming naturally immune. On the other
hand, what if the intervention simply allowed the children to
pass through an age window in which their susceptibility to
severe disease and death, being driven by innate age-related
factors, peaked? An examination of outcomes with such inter-
ventions indeed favors the former interpretation. Rebound
morbidity was not observed in several large-scale placebo-con-
trolled trials (186, 194), although one such trial noted relatively
slight rebound anemia among those treated (170). A recently
published study examined this question in detail (8). A 4-year
follow up of Tanzanian children receiving weekly prophylaxis
with Deltaprim or a placebo between ages 2 and 12 months
suggested that the reduction of exposure to P. falciparum early
in life delayed the acquisition of immunity. However, those
authors argued that,
Our data do not support this conclusion [that inter-
mittent therapy sets up children for more severe dis-
ease later in life], but instead suggest that a delayed
acquisition of immunity may lead to a small, but not
FIG. 3. Age- and exposure-dependent inversion of susceptibility to disease. These graphs illustrate the apparent age-dependent inversion of
susceptibility to death caused by P. falciparum with acute (a) versus chronic (b) exposure. Malaria-naive travelers experiencing acute exposure to
infection show a sharp increase in the risk of death (odds ratio) with increasing age, whereas the mortality rate for malaria among people living
in an area of holoendemic transmission shows the opposite trend. (Reproduced from reference 11 with permission of the Liverpool School of
FIG. 4. Hypothetical basis of age-dependent inversion of suscepti-
bility to disease with acute versus chronic exposure in children and
adults. Consider Th1- and Th2-type immune responses as surrogates
for any immune response that changes with age independent of expo-
sure and plays a critical role in infection outcomes. Th1-driven effec-
tors may dominate the immune response of children (hollow arrows),
whereas Th2-driven effectors may dominate the adult immune re-
sponse (solid arrows). These distinct, age- and exposure-dependent
responses cause harm or benefit to the host. (Reproduced from ref-
erence 11 with permission of the Liverpool School of Tropical Medi-
22 DOOLAN ET AL.CLIN. MICROBIOL. REV.
significant, increase in the cumulative number of ma-
laria episodes and, importantly, to a lower CR [cu-
mulative rate] of severe malaria. A similar pattern
was observed for severe anaemia, which is the other
major life-threatening complication of malaria in
young children. In summary, shifting the age pattern
of disease to older age groups does not markedly
affect the overall number of mild uncomplicated fe-
brile episodes or lead to an increase in severe ma-
laria, and it is associated with an overall decreased
CR of severe anaemia (8).
We agree with those authors and point to age-dependent
(versus cumulative exposure-dependent) NAI as the likely ba-
sis of their key observation.
ACQUISITION OF NAI
The molecular and cellular events that drive the onset of
immunity against malaria constitute the crux of efforts to ob-
tain an understanding of sufficient depth for rational exploita-
tion in the development of vaccines. Two fundamentally dis-
tinct hypotheses have emerged (Table 3). The most widely
accepted hypothesis explains the slow onset of clinical immu-
nity in populations where the disease is holoendemic on the
basis of parasite diversity. In brief, NAI is viewed as the cu-
mulative product of exposure to multiple parasite infections
over time, yielding a sufficiently diverse repertoire of strain-
specific immune responses. In areas of holoendemicity in Af-
rica, the onset of clinical immunity requires 10 to 15 years of
roughly five infections per year. The alternative hypothesis,
based largely on the observations of transmigrants in Indone-
sian Papua, attributes the onset of clinical immunity to recent
heavy exposure and development of cross-reactive, strain-tran-
scending immune responses governed predominantly by intrin-
sic characteristics that change with age independent of lifelong
parasite exposure. At the core of the disagreement between
these hypotheses is the basis of susceptibility/resistance to fal-
ciparum malaria: it is driven by the extrinsic factor of antigenic
diversity on the one hand or by the intrinsically appropriate/
inappropriate age-dependent immune responses on the other.
The distinction is not simply academic; it defines vaccine de-
velopment strategies that seek to overcome the perceived in-
adequate exposure to antigenic diversity or to overcome an
intrinsically inappropriate immune response to sufficient expo-
sure to antigenic diversity. In the context of modeling NAI in
vaccine development, this is a crucial point.
Within a single species of Plasmodium, allelic polymor-
phisms result in the coexistence of different genotypes, i.e.,
clones or so-called “strains” (167, 315). The definition of the
term “strain” has recently been extensively reviewed (209).
Allelic polymorphisms or genetic polymorphisms in certain
protein loci give rise to antigenically distinct forms of the
protein in different parasite clones or strains. This type of
diversity (antigenic diversity/antigenic polymorphism/strain
heterogeneity) underlies the concept of strain-specific immu-
nity (98, 125). According to this hypothesis, immunity to P.
falciparum is essentially strain specific (49, 318), and thus an
TABLE 3. Strain-specific and strain-transcending hypotheses for development of NAI
Basis of susceptibility
Evidence of basis of
in 5–15 yr
Early childhood morbidity
and mortality in areasof holoendemicity
to heterologous strain challenge ofa homologousimmunity
(possible confounding byIgM, a superior agglutinin)
Rapid 3–5 infections
in 1–2 yr
Recent exposure Age of host
Onset of NAI among
nonimmune adult migrants to areas ofhyperendemicity areas within 1–2 yr
Relative resistance to
heterologous strainchallenge of a broader immunity
protection mediated byIgG, a poor agglutinin
VOL. 22, 2009ACQUIRED IMMUNITY TO MALARIA23
individual becomes immune after being exposed to a large
number of strains circulating in the community. Evidence for
strain-specific immunity has accumulated from animal studies,
in particular experimental infections in monkeys (57, 164), as
well as induced infections in humans, including the treatment
of syphilis patients by inoculations of Plasmodium (67). In
experimental malaria in humans, a primary infection by one
parasite strain elicited an immune response which was capable
of protecting against that strain but not against infection by a
different strain (161). In Aotus monkeys, repeated infections
induced increasingly more rapid sterile immunity to homolo-
gous challenge (164).
Such studies revealed an important difference between the
clinic and the field: the onset of immunity was apparently rapid
in the neurosyphilis patients and slow in the field. The rapid
onset of experimentally induced immunity was attributed to
the use of homologous strains of parasites, and the slow onset
in areas of endemicity was attributed to heterologous chal-
lenge. The immune system was thought to require continuous
exposure to many heterologous strains in order to expand the
repertoire of relevant memory effector cells required to effec-
tively suppress subsequent challenges with antigenically di-
verse parasites in natural populations. However, the neu-
rosyphilis patients who were repeatedly challenged did not
include children, for obvious reasons. If the onset of clinical
immunity in a hypothetical set of serially challenged children
was not rapid, antigenic variation would not explain a slow
versus rapid onset of clinical immunity. Likewise, if naturally
exposed nonimmune adults rapidly acquired clinical immunity,
a strain-transcending immune response would seem the most
likely explanation. This was the key analytical insight offered by
the populations of transmigrants in Papua.
The concept of phenotypic antigenic variation is distinct
from that of strain-specific immunity. Broadly, the term anti-
genic variation refers to changes in antigenic phenotype by
regulated expression of different genes of a clonal population
of parasites over the natural course of an infection. Indirect
evidence for antigenic differences between parasite popula-
tions was first reported by Cox (91), when mice harboring
chronic infections with P. berghei were demonstrated to be
more susceptible to challenge with relapse parasites than to
challenge with the original parasite population. The classic
demonstration of antigenic variation in P. knowlesi by Brown
and Brown in 1965 (47), when chronic erythrocytic infections
of P. knowlesi were shown to consist of a succession of anti-
genic variants, provided the foundation for studies of antigenic
variation and evasion of the host immune response as an ex-
planation for the apparently slow onset of NAI. In this sce-
nario, the Plasmodium parasite evades host immunity by vary-
ing the antigenic character of infected erythrocytes. Consistent
with this, a large and extremely diverse family of P. falciparum
genes, known as the var genes, have been described (25, 287,
294). These encode an antigenically diverse parasite-derived
protein of 200 to 350 kDa, P. falciparum erythrocyte membrane
protein 1 (PfEMP1), on the surface of parasitized erythrocytes
with the expected properties of antigenically variant adhesion
molecules. PfEMP1 has been implicated as the key target an-
tigen involved in NAI to malaria (108, 152, 175). More re-
cently, two other larger families of clonally variant surface
molecules called rifins (115, 179) and STEVOR (166) have
been described, although their role in acquired immunity re-
mains unknown. Collectively, these proteins, expressed at the
infected red blood cell membrane, are referred to as variant
surface antigens (VSA), and the immunity directed against
these antigens is termed variant-specific immunity.
It has been demonstrated that VSA of P. falciparum can
undergo clonal variation in vitro to a variety of antigenic and
adhesive phenotypes in the absence of immune pressure (255).
Furthermore, it has been demonstrated that parasites of a
given strain can undergo antigenic variation in vivo, as shown
in monkey models by a switch in the antigens exposed on the
erythrocyte surface following transfer of erythrocytes infected
with a given P. falciparum strain from a splenectomized into an
intact Saimiri monkey (150). Following challenge, previously
infected spleen-intact Saimiri monkeys demonstrated sterile
immunity to the homologous parasite strain but not to a het-
erologous strain (148), although the peak parasitemia was per-
haps lower and of shorter duration following heterologous
challenge (244). In other nonhuman primate studies, passive
transfer of malaria-specific antibodies to a naive splenecto-
mized Saimiri monkey infected with P. falciparum resulted in
the emergence of parasites resistant to the transferred anti-
bodies, but monkeys primed with original parasites were fully
susceptible to challenge by the resistant ones, and vice versa
A number of studies have examined the role of antibody
responses to VSA in the development of NAI, by using tradi-
tional (agglutination) (191) as well as novel (flow cytometry)
immunological assays. VSA expressed during episodes of clin-
ical malaria in Kenyan children were less likely to be recog-
nized by the preexisting antibodies in the same child than that
by other children, as assessed by agglutination (52), and agglu-
tination by diverse plasma was associated with severe disease
and young host age (51). Anti-VSA IgG levels have been
correlated with protection from clinical malaria in Ghana (102,
232), Kenya (169), Gabon (324), and Tanzania (187). A recent
study with malaria-naive humans experiencing a single P. fal-
ciparum infection demonstrated antibody reactive with up to
six P. falciparum lines expressing different heterologous
PfEMP1 variants (112). Taken together, these studies give
support to the hypothesis that anti-VSA antibodies may pro-
vide variant-specific protective immunity (154–156), specifi-
cally immunity against severe disease (163), and VSA antigens
have been proposed for malaria vaccine development (63).
Strain-Specific versus Cross-Reactive (Strain-Transcending)
Both strain-specific immunity and cross-reactive immunity
have been documented in mice, monkeys, and humans in the
laboratory as well as the field.
Extensive antigenic diversity has been revealed using mono-
clonal antibodies assayed by the indirect fluorescent-antibody
test on laboratory and clinical isolates of P. falciparum schi-
zonts and merozoites from distinct geographical areas (85, 86,
94, 197–199). Significant strain-specific and cross-reactive in-
hibition of parasite multiplication was observed when homol-
24 DOOLAN ET AL.CLIN. MICROBIOL. REV.
ogous and heterologous sera from Gambian children were
assessed for any inhibitory effects on parasite growth (318).
Similarly, in nonhuman primates, it was demonstrated that
immune sera from Aotus monkeys contained antibodies that
blocked or reversed cytoadherence in vitro and that were iso-
late specific (305). Strain specificity of antibody recognition
and growth-inhibitory activity has been particularly well docu-
mented for one of the leading blood-stage vaccine candidate
antigens, P. falciparum AMA1 (77, 141, 143, 168, 219).
Like antigenic variation, antigenic diversity has been re-
vealed using agglutination assays. In humans, studies using
both field isolates and laboratory clones demonstrated that the
predominant agglutinating antibody response in humans was
variant specific, and antibodies which cross-react between dif-
ferent serotypes were rare (229). In The Gambia (190) and in
Papua New Guinea (118), sera from children in the convales-
cent stage of infection reacted with autologous but, with few
exceptions, not with heterologous infected cells in the anti-
body-mediated agglutination assay. No two isolates with the
same agglutinating phenotype could be identified in a group of
20 P. falciparum isolates from children in Papua New Guinea
(246) using children’s convalescent-phase sera or adult im-
mune sera. In nonhuman primates, chronic erythrocytic infec-
tions of P. knowlesi were shown to consist of a succession of
antigenically distinct strains (47), sera from rhesus macaque
monkeys immune to one P. knowlesi strain did not agglutinate
cells infected with another strain (46), P. knowlesi schizont-
infected erythrocytes did not agglutinate with sera from mon-
keys suffering a chronic infection with another strain (46), and
antibodies in sera from P. falciparum-infected Aotus monkeys
recognized antigenically diverse determinants, rather than con-
served epitopes, on the surface of infected erythrocytes (151).
Despite in vitro and in vivo support for the strain specificity
of immune responses, there is evidence that naturally exposed
individuals develop cross-reactive antibodies which recognize
an increasingly broad array of P. falciparum isolates with in-
creasing age or exposure. In The Gambia (190) and in Papua
New Guinea (118), although sera from children failed to ag-
glutinate heterologous infected cells in the agglutination assay,
adult immune sera contained antibodies that recognized by
agglutination the majority or all of the isolates and reacted
with the surface of infected cells from most children. Chatto-
padhyay et al. (62) demonstrated cross-reactive antibodies
against VSA among adults in an area of hyperendemicity in
India. An almost total lack of geographical specificity in such
agglutination assays was reported for both African and South
American adults (2). These data are consistent with the acqui-
sition of immunity against antigenically diverse strains with
It should be noted, however, that methodology may be a
potentially important confounder of the agglutination data.
None of the studies discussed here has used purified IgG,
which is thought to be the immunoglobulin of key importance
to NAI. Since IgM titers increase in responses to natural in-
fection and quickly wane (83) and IgG is a poor agglutinin
relative to IgM, the reported agglutination studies may be
biased by strain-specific IgMs that may have little relevance to
protective immunity. Moreover, it should be recognized that
IgG cannot act as an agglutinin in red cell suspensions that lack
sufficient concentrations of albumin to neutralize the zeta po-
tential of the surface of red blood cells. The zeta potential is a
negative charge that repels red blood cells at very close prox-
imity, and IgM but not IgG can bridge that distance. This
phenomenon would further favor IgM- over IgG-mediated ag-
glutination. Agglutination studies using purified IgG are
needed to corroborate the studies described above. Reeder et
al. (245) questioned the relevance of whole-serum agglutina-
tion studies with the finding that sera from transmigrant adults
and children exhibited restricted red cell agglutination profiles
relative to those of people native to Papua. The parasitologi-
cally demonstrated protection of the adult transmigrants was
not reflected in the results of the assays.
Strain-Specific versus Cross-Reactive
With regard to protective immunity, there are a number of
reports documenting a significant isolate-specific component in
the induced immunity against Plasmodium spp. both in non-
human primates and in humans.
In chimpanzees, it was shown that infection with a West
African strain of P. falciparum could protect against challenge
with the homologous strain of P. falciparum but was much less
effective against heterologous challenge (267). In splenecto-
mized gibbons, infection with P. falciparum isolates of Thai
origin conferred significant protection against homologous
challenge, and some isolates were able to stimulate a degree of
cross-protection similar to that conferred by the homologous
strain. However, other strains stimulated little or no cross-
protection (57). Likewise, Brown et al. (46) reported partial or
complete homologous variant immunity to P. knowlesi in rhe-
sus monkeys following infection (P. knowlesi-infected erythro-
cytes) and drug cure which was not as effective against a het-
erologous variant challenge. Voller and Rossan (313) tested
heterologous antigenic variants (from a homologous strain)
and demonstrated complete protection despite antigenic dis-
similarity; i.e., variant-transcending immunity appears to be
the rule. Fandeur and Chalvet (113) demonstrated both vari-
ant- and strain-specific immunity against P. falciparum in Aotus
monkeys but also demonstrated the apparently strict require-
ment for having only a single precedent infection. When ani-
mals had more than a single variant or strain infection expe-
rience, immunity to challenge proved to be both variant and
strain transcending. In all models, the degree of protection
against heterologous challenge increased following multiple
reinfection with a homologous strain (57, 286).
In humans, it was demonstrated that protective immunity
against heterologous challenge with distinct strains of P. vivax
(39, 67, 158, 161), P. cynamolgi (227), and P. falciparum (38,
160, 161) was less effective than that against homologous
strains. Furthermore, the degree of protective immunity in-
duced by malariatherapy against challenge with heterologous
strains was almost always less effective than that against ho-
mologous challenge (38, 41, 161, 243). Nonetheless, almost all
of these studies demonstrated a conspicuous partial protection
with homologous immunity to heterologous challenge. The
data from Powell et al. (243) seem typical, where the mean
percent reductions in days with relatively high parasitemia,
mild fever, or high fever between the first and second challenge
with either homologous (14 human subjects) or heterologous
VOL. 22, 2009ACQUIRED IMMUNITY TO MALARIA25
(5 human subjects) strains were barely distinguishable (Fig. 5).
Although homologous immunity performed “better,” with
slightly higher reductions in markers of infection susceptibility,
clinical protection from heterologous challenge was, as the
authors said, “unequivocal.” Among subjects challenged re-
peatedly with homologous strains and then challenged with a
heterologous strain, parasitemia initially behaved as if the per-
son was nonimmune, complete with high parasite counts and
fever (243). However, after 9 days the parasitemia spontane-
ously fell and fever subsided. These subjects retained control
over these parasitemias and remained asymptomatic for a
month or more until therapy was finally administered (243).
The onset of strain-transcending protective immunity was, as
the authors pointed out, “rapidly acquired.”
Other evidence of strain-transcending clinical immunity
comes from a wide range of other experiments. No difference
was observed between the infections developed by residents of
an area of hyperendemicity in Liberia experimentally infected
with P. falciparum sporozoites from a local strain and those
infected with a strain from geographically distinct areas, al-
though there was no evidence that these strains were in fact
distinct (43). When neurosyphilis patients who had been in-
fected with P. falciparum-parasitized erythrocytes of Colom-
bian origin 18 months previously were infected with a Thai
strain, parasitemia spontaneously cleared, concomitant with a
marked increase in antibody specific for the Colombian strain
and a much slower induction of lower-titer antibodies specific
for the Thai strain (81). A marked reduction in parasitemia
was observed following passive administration of IgG from
immune adult West Africans to children resident in East Af-
rica (205) or Thailand (266), to Aotus monkeys infected with an
Asian isolate of P. falciparum (101), or to Saimiri monkeys
acutely infected with P. falciparum strains of African or French
Guyana origin (137). Volunteers immunized with irradiated P.
falciparum sporozoites were protected against challenge with
heterologous P. falciparum sporozoites (69, 70, 250). Finally, in
the landmark study of clonal antigenic variation in P. knowlesi,
Brown and Brown (47) recognized a significant strain-tran-
scending immune response. This was also noted for P. fragile in
toque monkeys (139). The relevance of strain-specific versus
strain-transcending aspects of the immune response to NAI
has not been elucidated.
The burden upon advocates of strain-specific immunity is
the demonstration that a less pronounced protection following
heterologous challenge in hosts with homologous immunity
has relevance to an immune system that sees a dozen or more
infections in just 2 years. Interpreting slightly less effective
protective responses to heterologous challenge after a single
exposure to infection (Fig. 5) as evidence of the strain speci-
ficity of NAI seems a narrow view. Validation of strain speci-
ficity as the dominant factor governing the seemingly slow
onset of NAI surely requires stronger and more direct evi-
dence. For example, do serial challenges with five distinct
strains leave the subject susceptible to challenge with a sixth
strain? This has not been demonstrated to be true and seems
doubtful in light of a broader view of the data from the het-
erologous variant and strain challenge studies in human and
animal subjects cited above. No direct evidence supports the
supposition that strain-specific immunity represents a domi-
nant factor in host susceptibility to infection by P. falciparum as
occurs in areas of heavy exposure.
Indeed, a very persuasive argument that the protective im-
munity against malaria is predominantly strain transcending
rather than strain specific may be made. Intuitively, the num-
ber of episodes of clinical malaria experienced by a given
individual even in areas of high endemicity must be signifi-
cantly smaller than the repertoire of antigenic types. This may
be especially true in light of the apparently vast potential for
variation contained in the var genes of a single clone of P.
falciparum. The issue of persistent variant- or strain-specific
immunity must also be considered: if even just strain-specific
NAI accumulates through the decade of childhood, what ac-
counts for the apparent persistence of immunity to strains seen
long ago? The requirement for uninterrupted exposure for
maintenance of NAI has long been and is still widely acknowl-
edged, and recent studies tend to affirm the view. Although
Africans taking up long-term residence in France showed less
susceptibility to falciparum malaria than natives of Europe,
these were all people reporting to hospital with illness, 4.4% of
whom had severe disease (35). A similar study in the United
Kingdom showed that 31% of Africans reporting to hospital
with illness required parenteral treatment and that 4.3% re-
quired management in an intensive care unit (ICU) (compared
to 41% and 15% of native French, respectively) (53). Jennings
et al. (162) found that “… severe disease was no less common
among patients who might be assumed to have a degree of
protective immunity than among previously malaria-naïve pat-
ents.” Given the apparently short-lived nature of protection
afforded by NAI, it seems unlikely that protection against a
given strain seen in childhood could effectively persist through
A relatively rapidly acquired strain-transcending immunity
in older children and adults would explain much that appears
to be mysterious about the accumulation of a strain-specific
immunity over years. How is it that adults lose NAI, apparently
quickly and at least temporarily, when they leave areas of
endemicity when they are supposedly carrying protective im-
munity to strains experienced in early childhood? How did
Javanese adults, who were demonstrably susceptible to severe
disease and death at the onset of exposure, develop clinical
FIG. 5. Onset of clinical immunity to falciparum malaria in a sec-
ond experimental challenge with homologous versus heterologous
strains. Solid bars show the percent reduction in number of days with
greater than 1,000 parasites per ?l after challenge with a homologous
(left, data from 14 human subjects) or heterologous (right, data from
5 human subjects) strain. Gray bars show the percent reduction in
mean number of days with a fever higher than 100°F. Light bars show
the same for a fever higher than 102°F. (Based on data from reference
26 DOOLAN ET AL.CLIN. MICROBIOL. REV.
immunity to heterologous (wild-type) strains after just four
infections over 2 years? Why do humans and animals exposed
to serial challenge almost uniformly exhibit variant- and strain-
transcending immunity and at least suppress heterologous vari-
ants or strains even with just a single exposure? Given what we
now understand of VSA in P. falciparum, how is that neu-
rosyphilis patients undergoing malaria therapy developed solid
clinical immunity after just four challenges with allegedly ho-
mologous strains? Is the repertoire of variants within strains so
limited, or are these switches simply defeated by host immunity
with relative ease? We consider these questions incompatible
with the model of cumulative strain-specific immunity and con-
sistent with an immunity driven by only relatively recent expo-
sure and the intrinsic nature of the immune response in the
physiologically mature. Thus, we consider the basis of age-
dependent and chronic-exposure-independent NAI the key to
developing an understanding of NAI sufficient to exploit in
rational vaccine development, as well as safely attacking ho-
loendemic malaria transmission.
EMPIRICAL OBSERVATIONS OF PREERYTHROCYTIC-
The induction of stage-specific immunity against the
preerythrocytic stage of Plasmodium parasites was first dem-
onstrated in 1941 by Mulligan and coworkers (226). Repeated
injections of UV radiation-inactivated P. gallinaceum sporozo-
ites rendered chickens partially immune to sporozoite chal-
lenge, as assessed by a decrease in mortality and an increase in
the proportion of animals that spontaneously recovered from
infection. In 1948, a neurosyphilis patient previously treated
with malaria therapy was bitten by 2,000 infectious P. vivax-
infected mosquitoes and intravenously injected with the sali-
vary gland emulsion of 200 others and failed to develop
detectable blood-stage parasitemia or clinical disease; preeryth-
rocytic forms of the parasite were readily detected in the liver
(89). In 1966, Richards (248) demonstrated that protective
immunity against P. gallinaceum sporozoite challenge could be
induced in chickens by active immunization with sporozoites
inactivated by either UV irradiation, drying, freeze-thawing, or
formalin treatment. Holbrook et al. (147) obtained some
protection in mice against P. berghei by immunization with
formalin-treated exoerythrocytic stages of P. fallax.
Recent studies have focused on the induction of whole-
organism immunity by immunization with Plasmodium sporo-
zoites attenuated by radiation. Gamma irradiation of Plasmo-
dium-infected mosquitoes attenuates the parasite such that it
can invade the host hepatocyte but fails to differentiate into
erythrocytic stages (281). It is now well established that immu-
nization with radiation-attenuated sporozoites induces sterile
protective immunity against malaria in every model studied,
including humans, nonhuman primates, and rodents. Immuni-
zation with irradiated P. berghei, P. chabaudi, or P. yoelii sporo-
zoites protects against sporozoite challenge in rats and mice
(145, 230, 273, 316). Immunization with irradiated P. cynamolgi
or P. knowlesi sporozoites protects monkeys (79, 84, 135), and
immunization with irradiated P. falciparum or P. vivax sporo-
zoites protects humans (69–71, 109, 110, 142, 144, 250, 251).
Protection is species specific but not strain specific (110, 144)
and may last for up to 9 months in humans (based on n ? 1)
Protection against malaria liver stages has also been dem-
onstrated after vaccination with live sporozoites under chloro-
quine treatment in mice (33), with heat-killed sporozoites
(138), and with genetically attenuated sporozoites (223, 224,
EMPIRICAL OBSERVATIONS OF ASEXUAL
The induction of asexual erythrocytic-stage immunity in all
species tested, including humans, monkeys, and mice, has also
been demonstrated. It was first reported in 1929 (296) that a
massive dose of parasitized erythrocytes injected into birds
with a latent or low-grade P. cathemerium blood-stage infection
entirely or almost completely cleared the infection within 1
day, whereas the same dose injected into a normal bird fre-
quently was fatal. In 1936, it was reported that whole blood
from highly immune subjects produced beneficial results when
injected into patients suffering from acute attacks of malaria
(293). In 1945, Freund et al. (120) induced strong protection
against P. lophurae in ducks by immunization with formalin-
killed erythrocytic stages in adjuvant. Formalin-killed merozo-
ites in adjuvant were shown to induce stage-specific protection
against P. fallax in turkeys (147), and McGhee et al. (204)
obtained complete protection in chickens with P. gallinaceum
erythrocytic stages. In mice, partial protection was achieved by
vaccination with inactivated (cryopreserved and homogenized)
P. chabaudi merozoites in complete Freund’s adjuvant (149).
The success of vaccination against the erythrocytic stages of
Plasmodium was extended to the nonhuman primate model.
Rhesus macaque monkeys were vaccinated with whole forma-
lin-killed P. knowlesi schizonts (121, 297) or fractions of these
parasites (282) in complete Freund’s adjuvant. Brown et al.
then demonstrated that immunization of rhesus monkeys with
freeze-thawed schizonts in adjuvant conferred protection
against homologous as well as heterologous P. knowlesi chal-
lenge (48). It was shown that such immunity could be induced
after as few as two immunizations (271). Subsequently, immu-
nization of rhesus monkeys with P. knowlesi merozoites emul-
sified in adjuvant induced sterilizing immunity which was spe-
cies specific but not strain specific (54, 56, 72, 215, 216). One
Kra macaque monkey immunized with P. knowlesi merozoites
in adjuvant was completely protected in four of five challenges,
while a monkey immunized in parallel with P. knowlesi schi-
zonts in adjuvant was not protected (215). With P. falciparum,
Voller and Richards observed a significant delay in the
prepatent period following immunization of Aotus monkeys
with formalin-treated schizonts of P. falciparum in adjuvant,
but the induced immunity was not sufficient to protect the
monkeys against death (312). Partial protection against homol-
ogous as well as heterologous P. falciparum challenge was
induced in Aotus monkeys by immunization with P. falciparum
merozoites in adjuvant (279, 280). Immunization with mature
schizont stages of P. falciparum conferred strong protection
against homologous challenge in Saimiri monkeys (106), and
Saimiri monkeys develop significant, long-lasting protection
(up to 7 months) against challenge with homologous as well as
VOL. 22, 2009 ACQUIRED IMMUNITY TO MALARIA27
heterologous strains of P. falciparum following infection with
Irradiated erythrocytic-stage parasites, in the absence of ad-
juvant, induced partial protection against P. gallinaceum in
chickens (61, 248), against P. lophurae in ducks and chickens
(253), and against P. yoelii nigeriensis in mice (314). Richards
also demonstrated that protective immunity against homolo-
gous P. gallinaceum challenge in chickens could be induced by
active immunization with erythrocytic-stage parasites inacti-
vated by either UV irradiation, freeze-thawing, or formalin
treatment but not drying (248).
The partial protection observed with irradiated blood-stage
parasites contrasted with the complete sterile immunity in-
duced by immunization with irradiated sporozoites in rats and
mice (231, 317) and in humans (70, 71, 250). Clearance of
parasites passively transferred into a neurosyphilis patient suf-
fering an acute attack of P. vivax (39) nevertheless demon-
strated that protective immune responses against asexual
erythrocytic-stage parasites can be rapidly activated.
More recently, a new whole-cell vaccination strategy using
ultra-low doses of nonattenuated Plasmodium-infected eryth-
rocytes (128, 322) has proven to induce protective immunity
which correlates with Th1 cellular immune responses rather
than antibody responses in mice (P. chabaudi) (111) and with
robust T-cell-mediated immune responses in humans (P. fal-
EMPIRICAL OBSERVATIONS OF TRANSMISSION-
It is the sexual stages of the malaria parasite, known as
gametocytes, which are responsible for the transmission of the
parasite to the mosquito vector. Effective transmission-block-
ing immunity was first demonstrated by immunization with
whole killed P. gallinaceum parasites (a mixture of asexual- and
sexual-stage parasites) in chickens or with P. fallax parasites in
turkeys (153). These studies were subsequently confirmed us-
ing purified sexual-stage gametes of P. gallinaceum in chickens
(59), P. knowlesi in rhesus monkeys (136), and P. yoelii in mice
(211). Other studies showed that gametocyte infectivity and
oocyst development of P. gallinaceum could be reduced or
eliminated in mosquitoes by immunizing the chickens on which
the mosquitoes feed with infected red blood cells that have
been inactivated with formalin or X rays (134). Effective trans-
mission-blocking immunity eliminating or reducing the infec-
tivity of malarial parasites for mosquitoes has been observed
following natural infections in humans, during infections in
animals, and following artificial immunization of animals with
sexual-stage malaria parasites (36, 60, 180). Recently, for ex-
ample, sera from Macaca mulatta monkeys (78, 218) or Aotus
lemurinus griseimembra monkeys (9) immunized with a leading
candidate for a transmission-blocking vaccine, recombinant
Pvs25, or from rabbits immunized with recombinant Pfs25
(218) were shown to have transmission-blocking activity in a
mosquito membrane-feeding assay, and this activity could be
correlated with the antigen-specific antibody titer by enzyme-
linked immunosorbent assay.
Naturally acquired sexual-stage immune responses (105), the
role of sexual-stage parasites and transmission-blocking immunity
in the development of NAI (36), and the public health implica-
tions of transmission-blocking vaccines (268) have been recently
develop, which presumably reflects the fact that only a relatively
small proportion of gametocytes are taken up by the mosquito
vector, the majority being cleared by the human host (36), but
there are few data to indicate whether or not sexual-stage specific
immunity increases with age (105). Epidemiological data show
that the prevalence of gametocytes declines with age (228), and
this was originally interpreted to be a direct consequence of in-
creasing immunity against asexual blood-stage parasites (184).
More recent data, however, show that the prevalence of gameto-
cytes decreases more rapidly with age than that of asexual-stage
parasites (reviewed in reference 105), indicating that immunity to
gametocytes develops more quickly than immunity to asexual
blood-stage parasites. NAI does not appear to inhibit gametocy-
togenesis, since immune adults retain the capacity to infect mos-
quitoes and constitute a sizeable and important part of the avail-
able gametocyte reservoir (225). Conversely, sexual-stage-specific
immune responses do not play a significant role in NAI. In the
classic antibody transfer experiments (75, 76), there was no ap-
parent effect of gamma globulin on gametocytes, unlike the dra-
matic effect on asexual blood-stage parasites.
STAGE SPECIFICITY OF NAI
The underlying mechanisms and antigenic specificity of pro-
tective immunity against malaria are not well understood; our
current knowledge base will be reviewed elsewhere. Epidemi-
ological data indicate that natural exposure to sporozoites does
not induce complete (sterilizing) antiparasite and antidisease
immunity (146) and that naturally acquired immune mecha-
nisms responsible for the acquisition of clinical protection in
areas of endemicity affect the prevalence of P. falciparum par-
asitemia, limiting the density of parasitemia and thereby de-
creasing the malaria-associated morbidity and mortality. The
widespread persistence of patent parasitemia in asymptomatic
individuals resident in areas where malaria is endemic and the
ability of hyperimmune serum to transfer protection suggest
that NAI is directed predominantly against the parasite in its
asexual erythrocytic stage and that it does not effectively block
sporozoite invasion or intrahepatic development. More specif-
ically, the primary targets of this protective immunity appear to
be the extracellular merozoites in circulation; the reduction in
parasitemia conferred by the passive transfer of immune serum
occurs following schizogony, and the serum does not appear to
be effective against developing or mature trophozoites or
against developing or mature gametocytes (73, 76, 100, 206).
These in vivo observations are confirmed by in vitro studies
with P. knowlesi (74) and P. falciparum (217, 238).
Although the preerythrocytic stage is an effective target of
induced immunity against malaria (104), we nonetheless be-
lieve it to be highly likely that this stage is also targeted by
protective immune responses during the development of NAI.
In the malariatherapy studies, infection was introduced either
by the bites of infected mosquitoes or by injection of infected
blood, and the prepatent period was dependent upon the in-
fectious dose. The immunity was thought to be stage specific
and directed only against asexual blood-stage parasites, regard-
less of whether the infection was induced by sporozoites or
blood-stage parasites since “the immunity does not prevent an
28DOOLAN ET AL.CLIN. MICROBIOL. REV.
appreciable although subclinical increase in the density of the
trophozoites” (40). Immunity nevertheless “limits the effective
multiplication [of the parasite] so they rarely, if ever, attain a
density sufficient to produce more than a slight clinical attack”
(40). However, it was concluded from earlier studies of in-
duced immunity (320) that immunity to P. cathemerium in
canaries was probably effective against sporozoites as well as
trophozoites, since canaries experiencing latent P. catheme-
rium blood-stage infections were able to completely clear the
parasites following inoculation with infectious sporozoites.
This inhibition of blood-stage parasitemia, together with the
delay in patency noted with some patients, is consistent with
acquired preerythrocytic immunity targeted at the infected he-
patocyte. Indeed, Sinton (284, 285), studying the induction of
protective immunity against sporozoites by immunization with
P. ovale in humans, concluded that immunity developed by
sporozoite immunization was more effective than immunity
developed as a result of blood inoculation. This view was sup-
ported by Russell et al. (263, 264), who observed that the
agglutinating antibody titer in chickens is often higher after
repeated infections with inactivated sporozoites than that in
acutely or chronically infected animals induced by blood inoc-
ulation. These data led those authors to speculate that it is
possible “that sporozoites are antigenically more potent than
Almost any endeavor in the field of immunity to malaria
represents a step in the journey to a practical and effective
vaccine that serves to diminish the enormous losses of life,
health, and prosperity imposed by these parasites. As with any
journey to an unfamiliar destination, the traveler must stop,
take bearings, and ask, “Am I going in the right direction?” We
consider the endeavor represented by this review to be such a
pause. The long chronological reach of this paper was not a
matter of academics. Our assessment of bearings reached the
point of origin by the necessity of addressing this question: how
did parasite antigenic variation come to dominate strategic
thinking in vaccine development? The answer, we believe, is
found primarily in three sets of observations: (i) the seemingly
slow onset of acquired immunity in areas of heavy transmis-
sion, where the effects of age and cumulative exposure could
not be separated; (ii) the rapid onset of acquired immunity in
neurosyphilis patients being reconciled to the slow onset in
settings of endemicity by the invocation of strain-specific im-
munity; and (iii) the discoveries of variant antigenic types, var
genes, and PfEMP1 proteins being held as the molecular basis
of the slow onset of strain-specific NAI in zones of heavy
Against this backdrop, the goal of vaccine development be-
came discovery of an antigen or set of antigens that could
mount a strain-transcending immune response capable of de-
feating the array of antigenic variation defenses mounted by
the parasite. A dominant role for antigenic variation in host
susceptibility to the parasite lies at the core of this strategic
thinking, based largely on the three sets of observations listed
In this review we have made an argument for an alternative
hypothesis for the basis of host susceptibility to falciparum
malaria. That hypothesis represents an almost complete depar-
ture from the conventional view, placing intrinsic differences in
how acquired immunity operates in immature and mature im-
mune systems as the basis of host susceptibility to malaria. The
genesis of this hypothesis may be found in observations that
challenged the earliest assumptions regarding the onset of
NAI. When a population of nonimmune migrants of all ages
became abruptly exposed to hyper- to holoendemic transmis-
sion of malaria, the adults developed clinical immunity after
three or four infections within 12 to 24 months. Their children
did not. These data upset two of the three foundations of
antigenic variation as the basis of host susceptibility. Onset of
immunity was not slow; it only seemed that way when age and
cumulative exposure could not be separated. A reexamination
of the strain-specific immunity described for the neurosyphilis
patients revealed substantial evidence of potent strain-tran-
scending immunity. The focus on the often relatively slight
differences between homologous and heterologous strain im-
munity in that body of work began to resemble a forced fit to
the hypothesis of antigenic variation as basis of host suscepti-
bility. Indeed, the onset of immunity in the Indonesian mi-
grants very closely resembled that among the neurosyphilis
We consider the body of evidence supporting intrinsic age-
related factors as the key determinants of host susceptibility to
falciparum malaria to be compelling. However, it is also in-
complete in terms of mechanistic detail and, therefore, con-
clusive proofs. The literature demonstrating age-related differ-
ences in host responses and susceptibility to infectious diseases
is as vast as it is heterogeneous. None of the many systems
studied, including falciparum malaria in humans, provides a
testable hypothesis of cellular and molecular mechanisms that
could account for the age-related phenomena observed in or-
ganisms and populations. There remains no understanding of
what may be at work in establishing the often very conspicuous
differences between immature and mature animals coping with
challenge by infection.
One likely starting point for more firmly establishing age-
dependent, strain-transcending immunity in malaria and for
beginning to sort through the myriad possible mechanisms at
work may be a nonhuman primate model. In particular, the
system of P. knowlesi in rhesus macaques offers important
advantages. The course and consequence of P. knowlesi infec-
tion in that monkey species resemble the hyperparasitemia and
death caused by P. falciparum in humans. Breeding colonies of
rhesus macaques may offer a sufficient cross-section of ages,
and the available immunological reagents for that animal
would permit relatively inexpensive, sophisticated, and imme-
diate work. If adult animals in that model prove to be more
susceptible to acute infection than younger animals and if
older animals experience more rapid and complete onset of
protective acquired immunity to P. knowlesi than younger an-
imals, the ground may be laid for penetrating analyses of sim-
ilar immune processes at work in humans.
Human populations offering the same analytical insights
rarely appear. In areas of heavy exposure, separation of the
cumulative effects of heavy exposure from those of intrinsic
age-related factors will be difficult. Nonetheless, Kurtis et al.
(178) achieved this by examining susceptibility to fever with
malaria relative to physiological markers of onset of puberty;
VOL. 22, 2009 ACQUIRED IMMUNITY TO MALARIA29
demonstrating the event (signaling physiological age) rather
than calendar age as the principal determinant of resistance to
fever with parasitemia. It may be possible to conduct similar
analyses by applying other physiological markers of age among
younger people. The work of Reyburn et al. (247), conducted
along a gradient of elevation and intensity of transmission
pressure, also offers analytical leverage against age versus ex-
posure. Another approach to teasing out the relative effect of
age versus exposure on the development of NAI in areas of
holoendemicity where both are superimposed is to design and
conduct intervention studies using targeted malaria tools, such
as IPT or continuous chemoprophylaxis, in defined age groups
or age-stratified cohorts (8).
Migration of nonimmune people of all ages into areas of
heavy endemicity occurs under some circumstances, as in the
studies from Indonesian New Guinea discussed here. The in-
creasing demand for natural resources and people striving for
improvement of their economic welfare will continue to push
development into less and less hospitable surroundings. These
migrations occur despite the risks to life from many causes,
including malaria. What is a normal hazard in the pursuit of
life hinges upon what is necessary to secure essential needs,
and people in a London suburb will have different standards
from those at the edge of an Amazonian forest. Provided that
a research agenda did not prompt the migration, the study of
the people who embark on journeys of high risk of their own
free will (298) imposes no ethical obstacles beyond those of
international standards for research involving human subjects.
Indeed, avoiding health research in such populations may be
ethically dubious without a compelling justification. A dislike
of the consequences of human migrations does not adequately
support assertions of ethical compromise in the conduct of
medical research within such populations.
This work was supported in part by a Pfizer Australia Research
Fellowship (to D.L.D.), as well as by support from the Wellcome Trust
and the Southeast Asian Clinical Research Network (to J.K.B.).
We express our sincere gratitude to the anonymous peer reviewers
of this work for their constructive criticisms.
We report no conflicts of interest with regard to this paper.
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