Dengue Epidemiology in Infants
• JID 2009:200 (15 December) • 1893
M A J O R A R T I C L E
Dengue Virus Infections and Maternal Antibody
Decay in a Prospective Birth Cohort Study
of Vietnamese Infants
Tran Nguyen Bich Chau,1Nguyen Trong Hieu,3Katherine L. Anders,1,5Marcel Wolbers,1,5Le Bich Lien,4
Lu Thi Minh Hieu,4Tran Tinh Hien,2Nguyen Thanh Hung,4Jeremy Farrar,1,5Stephen Whitehead,6
and Cameron P. Simmons1,5
1Oxford University Clinical Research Unit,
of Dengue Haemorrhagic Fever, Children’s Hospital 1, Ho Chi Minh City, Vietnam;
United Kingdom; and
2Hospital for Tropical Diseases,
3Department of Neonatology, Hong Vuong Hospital, and
5Centre for Tropical Medicine, University of Oxford, Oxford,
6Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health,
Dengue hemorrhagic fever can occur in primary dengue virus (DENV) infection of infants. The decay of
maternally derived DENV immunoglobulin (Ig) G and the incidence of DENV infection were determined in
a prospectively studied cohort of 1244 Vietnamese infants. Higher concentrations of total IgG and DENV-
reactive IgG were found in cord plasma relative to maternal plasma. Maternally derived DENV-neutralizing
and E protein–reactive IgG titers declined to below measurable levels in 190% of infants by 6 months of age.
In contrast,IgGreactive withwholeDENVvirionspersisteduntil12monthsofagein20%ofinfants.Serological
surveillance identified 10 infants with asymptomatic DENV infection for an incidence of 1.7 cases per 100
person-years. DENV-neutralizing antibodies remained measurable for ?1 year after infection. These results
suggest that whereas DENV infection in infants is frequently subclinical, there is a window between 4 and 12
months of age where virion-binding but nonneutralizingIgGcouldfacilitateantibody-dependentenhancement.
Dengue is a major public health problem in many trop-
ical and subtropical countries. World Health Organi-
zation data suggest that the disease burden of dengue
has risen exponentially in the last 20 years, and the
number of countries affected has increased 10-fold .
This striking emergence of dengue is associated with
substantial costs to both health care systems and pa-
tients , and it is highly likely that the true disease
burden is underestimated by data based only on hos-
pitalized cases 
Dengue virus (DENV) belongs to the Flaviviridae
Received 8 June 2009; accepted 29 July 2009; electronically published 13
Potential conflicts of interest: none reported.
Financial support: Wellcome Trust.
Corresponding author: Dr Simmons, Oxford University Clinical Research Unit,
Hospital for Tropical Diseases, 190 Ben Ham Tu, District 5, Ho Chi Minh City,
The Journal of Infectious Diseases
? 2009 by the Infectious Diseases Society of America. All rights reserved.
family and includes 4 serotypes. Each serotype is ca-
pable of causing severe disease, called dengue hemor-
rhagic fever (DHF). A well-defined risk factor for DHF
is sequential infections by 2 different serotypes [4–7].
Although most of the DHF disease burden is the result
of secondary infections in children and young adults,
DHF also occurs in primary DENV infection of infants
!1 year of age [8, 9]. When DHF occurs in infants, it
can be clinically challenging to manage, and the mor-
tality rate is higher than in older children .
Maternally derived immunoglobulin (Ig) G is likely
to play a central role in immunity and pathogenesis of
dengue in infancy. The presence of maternally derived
neutralizing antibody is presumed to explain the low
prevalence of symptomatic dengue in infants !3–4
months of age . Thereafter, subneutralizing levels of
maternally derived anti-DENV IgG might enhance
DENV infection in Fc receptor–bearing cells, an event
that could contribute to DHF . In support of this
hypothesis, neat plasma from healthy infants born to
dengue-immune mothers has been shown to enhance
by guest on December 28, 2015
1894 • JID 2009:200 (15 December) • Chau et al
virus infection in a manner that correlates with the age-related
case burden of dengue in infants .
The epidemiology of DENV infection in infants is not well
defined. The only previous prospective cohort study of infants
!1 year of age demonstrated an infection incidence rate of ∼2
cases per 100 person-years . In southern Vietnam the an-
nual exposure risk in children is ∼10% per annum [13, 14],
but the exposure risk in infants is unknown. A better under-
standing of the epidemiology of dengue in infants and the
incidence of symptomatic illness will help guide decisions as
to when dengue vaccines should be introduced to endemic
regions. In addition, prospective cohort studies of infants at
risk of dengue can address important questions about the
quantitative and qualitative features of maternally derived an-
tibody and its role in immunity or pathogenesis.
This study aimed to determine the incidence of dengue ex-
posure and disease in a cohort of infants followed up pro-
spectively from birth and to establish the kinetics of decay of
maternally derived dengue-reactive antibody during the first
year of life. The main findings are that maternally derived
DENV virion–binding IgG persists for longer in infants than
neutralizing antibody and that the kinetics of this decay are
consistent with an association between nonneutralizing mater-
nal antibody and the age-related burden of dengue in infants.
Furthermore, the incidence of DENV infection in a cohort of
infants was determined to be 1.7 cases per 100 person-years,
with all infections asymptomatic.
MATERIALS AND METHODS
Healthy pregnant women (
were enrolled at Hung Vuong Obstetric Hospital in Ho Chi
Women were eligible to enroll in the study if they were human
immunodeficiency virus negative, had singleton pregnancies at
137 weeks gestation, and lived in districts adjacent to Hung
Vuong Obstetric Hospital and Children’s Hospital 1. Demo-
graphic information was recorded at enrollment, and cord and
maternal blood samples were collected at birth. The infant and
mother were then invited to return to the Hung Vuong Ob-
stetric Hospital every 3 months until the infant reached 1 year
of age and then again at 18 months and 2 years of age. At each
study visit, information on health care–seeking behavior was
recorded, and a capillary or venous blood sample was collected
from the infant. In line with national guidelines, no infant was
vaccinated against Japanese encephalitis virus before his or her
first birthday. The study was approved by the ethics committees
of the Hospital for Tropical Diseases and HungVuongObstetric
Hospital and the Oxford Tropical Research Ethics Committee.
Surveillance for febrile illnesses in the study population.
Study participants were encouraged to visit Children’s Hospital
This was a prospective birth cohort study.
) in their last trimestern p 1244
1 if their infant developed a febrile illness. At the time of pre-
sentation, information on clinical symptoms and history was
recorded in case record forms. If dengue was a possible diag-
nosis, a laboratory investigation (nonstructural protein1[NS1]
and IgM testing) was performed. In addition, a passive surveil-
lance system was used to detect febrile illnesses in the study
population. At each follow-up visit, the mother was asked
whether her infant had experienced a fever since the last visit,
and, if so, what health care had been sought and what diagnosis
had been given. If the mother and infant did not attend follow-
up, this information was obtained by telephone interview.
Dengue serological testing.
and IgG enzyme-linked immunosorbent assays (ELISAs) were
performed as described elsewhere, using antigens to DENV and
Japanese encephalitis virus . Dengue indirect IgG ELISAs
were performed using a pool of recombinant DENV E (recE)
proteins from each serotype (kindly provided by Hawaii Bio-
tech). Briefly, ELISA plates (Maxisorb; Nunc) were coated with
a pool of recE proteins from DENV-1, -2, -3, and -4 (final
concentration, 1 mg/mL) overnight, washed and then blocked
by 2% bovine serum albumin. After washing, 2-fold serial di-
lutions of plasma, starting at 1:100, were added for 1 h, plates
were washed again, and secondary antibody (horseradish per-
oxidase [HRP]–labeled anti-human IgG) was added for 1 h.
After another washing, plates were developed using tetrameth-
ylbenzidine (TMB) substrate, and the absorbance was read at
490 nm. End point titers were defined as the reciprocal of the
dilution that yielded an optical density of 0.3 after subtraction
of the absorbance obtained for the blank control and uncoated
In the Panbio Dengue IgG Indirect ELISA (Panbio), purified
DENV-1-4 virions are coated directly onto the microwells of
the plate. To measure end-point titers of IgG against these
virions, serial 3-fold dilutions of plasma were added to each
well for 1 h. After 5 washings, DENV-reactive IgG molecules
were detected by incubation with HRP-conjugated anti-human
IgG for 1 h, and after another washing, they were detected by
TMB substrate. End point titers were defined as the reciprocal
of the dilution that yielded an optical density of 0.3 after sub-
traction of the absorbance obtained for the blank control and
Definition of laboratory-confirmed DENV infection.
laboratory-confirmed DENV infection was defined as a positive
IgM or IgG antigen-capture ELISA result or a 4-fold increase
in titer between serial samples in the recE protein indirect
Plaque-reduction neutralization test.
body titers were determined by a complement-enhanced,
plaque-reduction neutralization test (PRNT). Briefly, test
plasma was heat inactivated (56?C for 30 min), and serial 2-
fold dilutions beginning at 1:10 were prepared in serum-free
Dengue antigen-capture IgM
by guest on December 28, 2015
Dengue Epidemiology in Infants • JID 2009:200 (15 December) • 1895
Table 1. Anti–Dengue Virus (DENV) Antibody Titers in Maternal and Cord Plasma in 50 Infants
Maternal plasma samples
(n p 50)
(n p 50)
Neutralizing antibody (PRNT50)
Anti–recE protein IgG
Total human IgG
53 (!10 to 805)
30 (!10 to 408)
42 (!10 to 4616)
26 (!10 to 460)
titers were included, and samples with titers !10 were given a log10value of 0.5. For anti–recombinant E (recE) protein immunoglobulin (Ig) G, the end-point
limit of detection for indirect enzyme-linked immunosorbent assay was 100. GMT, geometric mean titer.
aP values determined with paired-sample t test of log10-transformed values.
bValues for total human IgG concentrations are given in milligrams per deciliter.
The limit of detection for the 50% plaque-reduction neutralization test (PRNT50) was 10; for comparisons between cord and maternal samples, all
medium containing 0.25% human serum albumin. Test virus,
diluted to 500 plaque-forming units/mL in medium containing
guinea pig complement (Cambrex) at a complement fixation
titer of 1:10, was added to equal volumes of diluted test serum
and incubated at 37?C for 30 min. Medium was removed from
monolayer cultures of Vero cells on 24-well plates, and 0.1 mL
of virus-serum mixture was transferred into duplicate wells.
After incubation for 60 min at 37?C, the wells were overlaid
with medium containing 1% methylcellulose and 2% fetal bo-
vine serum. Samples were incubated at 37?C for 4–5 days, and
plaques were visualized by immunoperoxidase staining, and a
50% PRNT (PRNT50) titer was calculated. We used the follow-
ing viruses: DEN1 Puerto Rico/94, DEN2 NGC prototype,
DEN3 Sleman/78, DEN4 814669. The limit of detection was a
titer of 10.
Geometric mean titers were calculated
to summarize DENV-reactive IgG populations in maternal and
cord plasma, and a paired t test was used to test the null
hypothesis of no difference in log titer between paired cord
and maternal samples. The incidence of DENV infection was
calculated as the number of seroconversions per 100 person-
years of follow-up, and confidence intervals (CIs) were deter-
mined based on the Poisson distribution. For each child, we
estimated the time required for IgG levels to reach half the
birth (cord) value, based on a linear regression of the child’s
detectable log-transformed measurements, which depended on
follow-up time. Children with undetectable anti-DENV IgG
values at their first follow-up visit (age 3 months) were treated
as having a half-life shorter than all other children. A CI for
the median population half-life was based on the Kaplan-Meier
estimator of the half-life distribution andits CI.Theproportion
of children with detectable IgG values over time was alsograph-
ically displayed. All analyses were performed with R software
(version 2.8.1; R Foundation for Statistical Computing). All
reported CIs are 2-sided 95% intervals, and tests were per-
formed at a 2-sided significance level of 5%.
infants were enrolled at Hung Vuong Obstetric Hospital, Ho
Chi Minh City, Vietnam, between September 2006 and August
2007. The majority of participants (96%) were resident in Ho
Chi Minh City districts 5, 6, 8, 10, 11, Binh Chanh, Binh Tan,
Tan Binh, and Tan Phu, and the average maternal age was 27
years (range, 17–45 years). The newborn male-female ratio was
1.06:1. The median gestational age was 39 weeks (range, 30–
43 weeks), and the mean birth weight was 3.1 kg (range, 2.5–
4.5 kg). Overall, 66% of all enrolled infants (819 of 1244)
returned for ?1 scheduled follow-up visit; 226 of (28%) 819
infants returned for 1 visit only, 143 (17%) of 819 for 2 visits,
166 (20%) of 819 for 3 visits, and 284 (35%) of 819 for all 4
follow-up visits. The reported use of bed nets was high in the
cohort, with 195% of infants reportedly sleeping under bed
nets at 3, 6, 9, and 12 months of age. There were no apparent
demographic or socioeconomic differences between infants
who did not return for any scheduled follow-up (
those who returned at least once (
Maternal antibody transfer.
characteristics of transplacental IgG transfer and the decay of
different populations of anti-DENV antibody, we randomly se-
lected 50 infants with plasma samples for all4scheduledfollow-
up visits (ages 3, 6, 9 and 12 months) and no serological evi-
dence of DENV exposure. The average maternal age of this
n p 425
) (data not shown).
To better understand the
n p 819
by guest on December 28, 2015
1896 • JID 2009:200 (15 December) • Chau et al
with measurable anti–DENV recombinant E (recE) protein immunoglobulin (Ig) G titers, 50% plaque-reduction neutralization test (PRNT50) titers, and
anti–DENV virion IgG titers. This analysis includes only those infants whose cord blood contained a measurable level of antibody. Lines showing the
decay of anti–DENV recE protein IgG titers (in 49 infants) and anti–DENV virion IgG titers (in 49 infants) are identical in each panel; the third line in
each panel shows the fraction of infants with measurable PRNT50titers for DENV-1 (
4 (; D). Ab, antibody; Nt Ab, neutralizing antibody.n p 36
Dynamics of anti–dengue virus (DENV) maternal antibody decay in infants. Plots show the fraction of infants at each follow-up time point
; A), DENV-2 (; B), DENV-3 (; C), or DENV-n p 38n p 48n p 46
group was 27.5 years (range, 20–35 years). The median ges-
tational age was 39 weeks (range, 35–41.5 weeks). In maternal
plasma, 36 of 50 mothers (72%) had neutralizing antibody
(PRNT50) against all 4 DENV serotypes, and 98% had neu-
tralizing antibody to least 1 serotype (Table 1). Total IgG con-
centrations in cord blood were significantly higher than the
corresponding maternal sample, which probably explains the
observation of significantly higher anti–DENVrecEproteinIgG
titers in cord blood than in maternal blood. PRNT50titers for
each DENV serotype were also higher on average in cord blood
than in maternal blood, and this difference was significant for
DENV-2 and DENV-3 (Table 1). This finding suggests active
transport of maternal IgG (irrespective of specificity) across the
Decay of maternally derived DENV-reactive IgG.
els of maternally derived antibodies against purified DENV
virions and pooled recE proteins and neutralizing antibodies
(PRNT50) were assessed in cord plasma and in each plasma
sample collected at 3, 6, 9, and 12 months of age. Figure 1
shows the fractions of infants having measurable levels of these
maternal antibody populations at different times during the
first year of life. By 6 months of age almost all infants had lost
IgG capable of binding DENV-1–4 recE proteins in an indirect
ELISA. At the same age, neutralizing antibodies to DENV-1
were undetectable, and neutralizing antibodies to DENV-2, -3,
and -4 were measurable in only 10% of infants (Figure 1).
Interestingly, levels of IgG to DENV-1–4 virions (indirect
ELISA) were measurable for longer, being detectable at 12
months of age in ∼20% of infants (Figure 1). The estimated
median half-life of anti–DENV virion IgG was 5.2 weeks (95%
CI, 4.8–6.5 weeks), but this finding should be interpreted with
caution, because it is based on 3-monthly sampling only. The
important finding here is that neutralizing antibody (at least
as measured by PRNT50assay) does not persist for as long as
antibodies that can bind the surface of DENV virions.
Serological detection of DENV infectionduringthefirstyear
Because maternal IgM does not cross the placental
barrier, the presence of DENV-reactive IgM in infant plasma
reflects recent infection. DENV IgM seroconversions occurred
in 9 of 819 infants (2111 unique samples). To complement
detection of DENV IgM, an indirect IgG ELISA employing
pooled recE proteins from DENV-1–4 was used to screen cord
and infant plasma samples. With this approach, it was possible
to detect a 14-fold increase in the end point titer of IgG to
by guest on December 28, 2015
Dengue Epidemiology in Infants • JID 2009:200 (15 December) • 1897
umented DENV Seroconversions
Results of Dengue Virus (DENV) Diagnostic Serological Tests in 10 Infants with Doc-
aIgM and IgG were identified with IgM and IgG antigen-capture enzyme-linked immunosorbent assay.
bEnd-point titers of IgG against pooled DENV-1–4 recombinant E proteins were measured with indirect enzyme-
linked immunosorbent assay. The limit of detection was 100.
Ig, immunoglobulin; NA, sample not available; +, positive; ?, negative.
recE proteins in 1 additional infant. A summary of the findings
from the serological investigations is shown in Table 2. Assum-
ing that the indirect DENV-1–4 recE protein IgG ELISA pro-
vided serological surveillance for DENV infection from birth
until the last blood sample was collected from each infant, then
the 10 DENV infections detected in 572.5 person-years of se-
rological surveillance yielded an infection incidence of 1.7 cases
per 100 person-years (95% CI, 0.8–3.2 cases per 100 person-
years). Most of the DENV infections(9of10)weredocumented
in the rainy season of May to November 2007. The timing of
DENV infection in case 812 could not be defined accurately
because of the 9-month gap between follow-up samples. One-
half of the infections occurred before the age of 6 months but
none before age 3 months.
Surveillance for febrile events in the cohort of 1244 infants.
There were 252 fever episodes in 206 infants. Fever without a
clinical diagnosis was reported in 192 of 252 febrile infants
(76%). No febrile infant in the cohort had laboratory-con-
firmed dengue. Among the 10 infants with serological evidence
of infection, in only 1 was there a report of a fever (undi-
agnosed) in the 3 months preceding the detection of DENV
IgM. These data suggest that the large majority of DENV in-
fections, and possibly all, were not clinically apparent.
Antibody responses after DENV infection.
virions (indirect ELISA) and neutralizing antibody (PRNT50)
were measured in serial plasma samples from 9 of the 10 infants
with DENV infections. Samples were obtained from infants
with DENV infections until they were 2 years of age. One infant
had insufficient samples for evaluation. In the plasma sample
preceding infection, 7 (78%) of 9 infants still had detectable
levels of IgG to DENV virions, and 5 (56%) of 9 had IgG to
IgG to DENV
recE proteins (Table 2). Neutralizing antibody to any serotype
was below the limit of detection in preinfection samples from
5 of 9 infants (Figure 2). Because all infections were asymp-
tomatic, it was not possible to determine the precise age of
infants at infection and therefore not possible to extrapolate
the titer of DENV-reactive IgG populations at that time.
Postinfection PRNT50patterns were often multitypic, though
in most cases there was clearly 1 serotype that dominated the
response (Figure 2). DENV-1 was inferred to be the infecting
serotype in 7 (78%) of 9 infants on the basis of the PRNT50
pattern after infection, consistent with the widespread preva-
lence of DENV-1 in southern Vietnam during 2006–2007 (un-
published results). DENV-2 and DENV-3 were inferred to be
the infecting serotypes in cases886 and 387,respectively(Figure
2J and 2N). Despite their immunological immaturity, all in-
fants, including those !6 months of age,hadrobustneutralizing
antibody responses that were long-lived, being measurable 11
year after infection in all 7 infants with available follow-up
samples (Figure 2). Four infants were reportedly vaccinated
against Japanese encephalitis virus between the ages of 18 and
24 months, but this did not appear to significantly affect either
neutralizing antibody or IgG titers to DENV virions (Figure
2). Collectively, these results imply that asymptomatic DENV
infections elicit robust and durable neutralizing antibody re-
sponses in infants and that these response are independent of
age at exposure.
Understanding the disease burden and epidemiology of dengue
in infants is important in guiding how and when dengue vac-
by guest on December 28, 2015
(Ig) G to DENV virions in serial plasma samples before and after infection. B, D, F, H, J, L, N, P, and R, 50% plaque-reduction neutralization test
(PRNT50) titers for neutralizing antibodies to DENV-1–4 in the same serial plasma samples. Samples at time 0 are cord plasma. Boxed numbers on x
axes indicate ages at which DENV IgM was first, and dashed lines represent assay limits of detection. JEV, Japanese encephalitis virus.
Antibody profiles in 9 infants with asymptomatic dengue virus (DENV) infections. A, C, E, G, I, K, M, O, and Q, Titers of immunoglobulin
by guest on December 28, 2015
Dengue Epidemiology in Infants • JID 2009:200 (15 December) • 1899
cines are best used, if and when they become available. The
incidence of DENV infection in this cohort was 1.7 cases per
100 person-years (95% CI, 0.8–3.2 cases per 100 person-years),
a substantially lower exposure rate than that reported for older
children in Vietnam [13, 14]. Furthermore, our results suggest
the vast majority of DENV infections in infants are likely to
be asymptomatic. Despite a low infection incidenceandaneven
lower (unmeasured) incidence of symptomatic infection, we
nonetheless demonstrated that decay of maternally derived
anti-DENV IgG occurs with kinetics that create conditions po-
tentially conducive for antibody-dependent enhancement
(ADE). The loss of neutralizing antibody several months before
the loss of IgG that can bind intact DENV virions might fa-
cilitate enhanced viral infection and predispose some infants
to symptomatic or possibly severe disease.
Among the population of women we studied, almost all
(98%) had serological evidence of previous DENV infection,
consistent with high seroprevalence in southern Vietnam [13,
14]. As in other studies [12, 16, 17], we found that total IgG
and anti-DENV antibody titers were higher in cord blood than
corresponding maternal titers. The important wider relevance
of this finding is that studies of infants with dengue that use
maternal anti-DENV titers as a surrogate of cord blood titers
[8, 18] will sometimes underestimate the infant’s real titer at
The decay of maternally derived DENV-reactive IgG, mea-
sured by PRNT50and hemagglutination inhibition assay (HIA),
has been described elsewhere in separate cohorts of healthy
Thai infants [12, 16]. These studies have shown incremental
decay of antibody at PRNT50or HIA, with neutralizing anti-
body declining below measurable levels in 80% of infants by
9 months of age . Our results are in agreement with the
findings of these studies and extend them by showing that the
kinetics of decay of recE-specific IgG correlate with levels of
neutralizing antibody but that IgG reactive with whole virions
is measurable for much longer. This could reflect the binding
of specific IgG to other antigens exposed on the viral surface
(eg, pre M/M) or the binding of antibodies to epitopes that
are conformationally not present in recE proteins. We excluded
the possibility that these findings reflected binding to NS1 pro-
teins by demonstrating that NS1 was not detectable on the
virion-coated wells of the assay plate.
The presence of IgG that can bind the surface of the virion,
but not neutralize it, may provide the environment for ADE
to occur in infants 4–12 months of age. Consistent with this
possibility, findings from our other studies have indicated that
plasma from 6-month-old infants mediates better DENV in-
fection enhancement in vitro than plasma from younger or
older infants . Collectively, the results described here and
elsewhere are consistent with a relationship between sub- or
nonneutralizing, maternally derived, virion-binding IgG and
the age-related hospitalized case burden of dengue in infants,
with ADE an obvious candidate to explain this relationship.
The incidence of DENV infection in the cohort of infants
described here was 1.7 cases per 100 person-years. A similar
infection rate (∼2 cases per 100 person-years) was observed in
a cohort of Thai infants !1 year of age  in whom almost
all infections were asymptomatic, as in the current study. The
incidence of infections in infants described here is much lower
than the incidence of dengue in older Vietnamese children
suggested by epidemiological surveillance in southern Vietnam
for 2007 (∼700 hospitalizations for dengueper100,000children
!15 years old; estimate based on data from [19, 20]) and by
other seroprevalence studies, the findings of which have sug-
gested an infection rate of ∼10% per annum. The basis for a
lower DENV infection incidence in infants may be related to
parental behavior that reduces exposure risk in infants. For
example, Vietnamese infants are typically swaddled in clothes
and/or blankets for the first months of life, and, as this study
showed, most typically sleep under bed nets. Collectively these
factors will serve to limit exposure to biting Aedes aegypti. It
is also conceivable, but difficult to demonstrate, that maternal
antibody can prevent infection outright so that infants do not
undergo seroconversion and are therefore not detected as case
patients by serological surveillance.
The DENV infections documented here were associated with
the induction of long-lived neutralizing antibodies that were
often multitypic but occasionally monotypic. Responses in in-
fants !6 months old were as robust as those in older infants,
suggesting that their inherent immunological immaturityinthe
B cell compartment was not a barrier to establishing a pool of
memory B cells, of which a subset maintain relatively constant
antibody levels .
Our results and those of others  revealing the relatively
low incidence of dengue in infants suggest that implementation
of dengue vaccines would be better directed at olderagegroups,
in which the incidence and prevalence of disease are much
greater. A vaccine that reduces DENV transmission in the com-
munity will benefit infants by reducing the risk of infection.
We thank the families of study participants and physicians and nurses
at Hung Vuong Hospital and Children’s Hospital 1 for their contribution
to the study. Ms Hoang Thi Sen is thanked for her tireless efforts in follow-
1. World Health Organization. Dengue and dengue haemorrhagic fever.
Geneva: World Health Organization, 2008.
2. Suaya JA, Shepard DS, Siqueira JB, et al. Cost of dengue cases in eight
countries in the Americas and Asia: a prospective study. Am J Trop
Med Hyg 2009;80:846–55.
3. Anderson KB, Chunsuttiwat S, Nisalak A, et al. Burden of symptomatic
by guest on December 28, 2015
1900 • JID 2009:200 (15 December) • Chau et al
dengue infection in children at primary school in Thailand: a pro-
spective study. Lancet 2007;369:1452–9.
4. Graham RR, Juffrie M, Tan R, et al. A prospective seroepidemiologic
study on dengue in children four to nine years of age in Yogyakarta,
Indonesia. I. Studies in 1995–1996. Am J Trop Med Hyg 1999;61:412–9.
5. Thein S, Aung MM, Shwe TN, et al. Risk factors in dengue shock syn-
drome. Am J Trop Med Hyg 1997;56:566–72.
6. Burke DS, Nisalak A, Johnson DE, Scott RM. A prospective study of
dengue infections in Bangkok. Am J Trop Med Hyg 1988;38:172–80.
7. Sangkawibha N, Rojanasuphot S, Ahandrik S, et al. Risk factors in
dengue shock syndrome: a prospective epidemiologic study in Rayong,
Thailand. I. The 1980 outbreak. Am J Epidemiol 1984;120:653–69.
8. Simmons CP, Chau TN, Thuy TT, et al. Maternal antibody and viral
factors in the pathogenesis of dengue virus in infants. J Infect Dis 2007;
9. Halstead SB, Lan NT, Myint TT, et al. Dengue hemorrhagic fever in
infants: research opportunities ignored. Emerg Infect Dis2002;8:1474–9.
10. Nguyen TH, Nguyen TL, Lei HY, et al. Volume replacement in infants
with dengue hemorrhagic fever/dengue shocksyndrome.AmJTropMed
11. Chau TN, Quyen NT, Thuy TT, et al. Dengue in Vietnamese infants:
results of infection-enhancement assays correlate with age-related dis-
ease epidemiology, and cellular immune responses correlate with dis-
ease severity. J Infect Dis 2008;198:516–24.
12. Pengsaa K, Luxemburger C, Sabchareon A, et al. Dengue virus infec-
tions in the first 2 years of life and the kinetics of transplacentally trans-
ferred dengue neutralizing antibodies in Thai children. J Infect Dis 2006;
13. Bartley LM, Carabin H, Vinh Chau N, et al. Assessment of the factors
associated with flavivirus seroprevalence in a population in Southern
Vietnam. Epidemiol Infect 2002;128:213–20.
14. Thai KT, Binh TQ, Giao PT, et al. Seroprevalence of dengue antibodies,
annual incidence and risk factors among children in southernVietnam.
Trop Med Int Health 2005;10:379–86.
15. Hang VT, Nguyet NM, Trung DT, et al. Diagnostic accuracy of NS1
ELISA and lateral flow rapid tests for dengue sensitivity, specificity and
relationship to viraemia and antibody responses. PLoS Negl Trop Dis
16. Watanaveeradej V, Endy TP, Samakoses R, et al. Transplacentally trans-
ferred maternal-infant antibodies to dengue virus. Am J Trop Med Hyg
17. Ventura AK, Ehrenkranz NJ, Rosenthal D. Placental passage of anti-
bodies to Dengue virus in persons living in a region of hyperendemic
dengue virus infection. J Infect Dis 1975;131(Suppl):S62–8.
18. Kliks SC, Nimmanitya S, Nisalak A, Burke DS. Evidence that maternal
dengue antibodies are important in the development of dengue hem-
orrhagic fever in infants. Am J Trop Med Hyg 1988;38:411–9.
19. Household living standards survey, 2006. Hanoi, Vietnam: General Sta-
tistics Office of Vietnam, 2006.
20. Pasteur Institute. Annual report of the 2007 activities and 2008 plan for
the dengue control program in Southern Vietnam. Ho Chi Minh City,
Vietnam: Pasteur Institute, 2008.
21. Siegrist CA, Aspinall R. B-cell responses to vaccination at the extremes
of age. Nat Rev Immunol 2009;9:185–94.
by guest on December 28, 2015