Elderly Subjects Have a Delayed Antibody Response and
Prolonged Viraemia following Yellow Fever Vaccination:
A Prospective Controlled Cohort Study
Anna H. Roukens1*, Darius Soonawala1, Simone A. Joosten1, Adrie ¨tte W. de Visser1, Xiaohong Jiang2,
Kees Dirksen3, Marjolein de Gruijter4, Jaap T. van Dissel1, Peter J. Bredenbeek2, Leo G. Visser1
1Department of Infectious Diseases, Leiden University Medical Center, Leiden, The Netherlands, 2Department of Medical Microbiology, Leiden University Medical Center,
Leiden, The Netherlands, 3Municipal Health Center Hollands Midden, The Hague, The Netherlands, 4Municipal Health Center Leiden, Leiden, The Netherlands
Background: Yellow fever vaccination (YF-17D) can cause serious adverse events (SAEs). The mechanism of these SAEs is
poorly understood. Older age has been identified as a risk factor. We tested the hypothesis that the humoral immune
response to yellow fever vaccine develops more slowly in elderly than in younger subjects.
Method: We vaccinated young volunteers (18–28 yrs, N=30) and elderly travelers (60–81 yrs, N=28) with YF-17D and
measured their neutralizing antibody titers and plasma YF-17D RNA copy numbers before vaccination and 3, 5, 10, 14 and
28 days after vaccination.
Results: Ten days after vaccination seroprotection was attained by 77% (23/30) of the young participants and by 50% (14/
28) of the elderly participants (p=0.03). Accordingly, the Geometric Mean Titer of younger participants was higher than the
GMT of the elderly participants. At day 10 the difference was +2.9 IU/ml (95% CI 1.8–4.7, p=0.00004) and at day 14 +1.8 IU/
ml (95% CI 1.1–2.9, p=0.02, using a mixed linear model. Viraemia was more common in the elderly (86%, 24/28) than in the
younger participants (60%, 14/30) (p=0.03) with higher YF-17D RNA copy numbers in the elderly participants.
Conclusions: We found that elderly subjects had a delayed antibody response and higher viraemia levels after yellow fever
primovaccination. We postulate that with older age, a weaker immune response to yellow fever vaccine allows the
attenuated virus to cause higher viraemia levels which may increase the risk of developing SAEs. This may be one piece in
the puzzle of the pathophysiology of YEL-AVD.
Trial Registration: Trialregitser.nl NTR1040
Citation: Roukens AH, Soonawala D, Joosten SA, de Visser AW, Jiang X, et al. (2011) Elderly Subjects Have a Delayed Antibody Response and Prolonged Viraemia
following Yellow Fever Vaccination: A Prospective Controlled Cohort Study. PLoS ONE 6(12): e27753. doi:10.1371/journal.pone.0027753
Editor: Daniel G. Bausch, Tulane School of Public Health and Tropical Medicine, United States of America
Received February 9, 2011; Accepted October 24, 2011; Published December 7, 2011
Copyright: ? 2011 Roukens et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This study was funded by the department of Infectious Diseases of the Leiden University Medical Center. The funders had no role in the study design,
data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: email@example.com
The live attenuated 17D yellow fever vaccine is regarded as one
of the safest and most effective vaccines . However, in
immunocompromized individuals yellow fever vaccination can
cause fatal adverse events [2,3]. A hampered immune response
could allow the vaccine virus to replicate unrestrictedly, leading to
vaccine-associated disease that resembles wild type yellow fever
(yellow fever vaccine associated viscerotropic disease, YEL-AVD).
YEL-AVD is fatal in 50% of cases . In the last decade, a series
of these serious and sometimes fatal adverse events following
yellow fever vaccination has been reported [5–11]. The risk of
YEL-AVD is increased for those with a history of thymectomy
, male gender  and with increasing age. For vaccinees of
60–69 years this risk is estimated to be 1:100.000 doses and for
vaccinees of $70 years it is 2.3–3.2:100.000, which is approx-
imately a 4 and 11 fold higher risk than the risk for young adults
[13,14]. The higher risk of YEL-AVD in elderly travelers has
resulted in a more restrictive policy towards vaccinating travelers
of 60 years and older, also advised by the World Health
Organisation and Centers for Disease Control en Prevention
[15–18]. In this group the risk of serious adverse events following
vaccination is weighed against the risk of infection, using disease
surveillance data of the WHO and reports of yellow fever
The biological mechanism for the association between adverse
events and older age has not yet been elucidated . Both innate
and adaptive immune responses wane with increasing age .
This may allow the attenuated vaccine virus more time to replicate
and cause adverse events in elderly subjects. In this study we
focused on humoral immunity, as this is considered to confer
protective immunity against yellow fever. We tested the hypothesis
that the adaptive immune response to yellow fever vaccine
develops more slowly in elderly than in young subjects.
PLoS ONE | www.plosone.org1December 2011 | Volume 6 | Issue 12 | e27753
The protocol for this trial and supporting checklist are available
as supporting information; see Checklist S1 and Protocol S1.
The protocol and consent forms were approved by the Dutch
Central Committee of Human Research (CCMO) and by the
Medical Ethical Committee of the Leiden University Medical
Center (LUMC) in the Netherlands. The trial was registered under
NTR1040 and ISRCTN42180653, (http://irsctn.org). Written
informed consent was obtained from each participant prior to
This study was conducted to determine whether the adaptive
immune response to yellow fever vaccine is slower to develop in
persons of 60 years or older compared with persons aged 18 to 40
years. Primary outcomes were the humoral response to yellow
fever vaccination, measured by Plaque Reduction Neutralization
Test (PRNT), and Yellow Fever 17D (YF-17D) viraemia after
vaccination, which was quantified by real time PCR (qRT-PCR).
Secondary outcomes were adverse events.
Study design and Participants
In this prospective controlled cohort study, participants were
recruited at the Travel Clinic of the Leiden University Medical
Center (LUMC), and Municipal Health Centers of Leiden and
The Hague, the Netherlands. Healthy volunteers aged between
18 and 40 years and eligible for inclusion into the control group
were invited to participate. Participants in the control group were
not necessarily planning to travel to a yellow fever endemic area.
The study group consisted of healthy travelers aged 60 years or
above, who had an indication for yellow fever vaccination based
on their travel destination (National Coordination Center for
Travelers’ Health, LCR) . Individuals who had previously
received yellow fever vaccine or who had a compromised
immunity due to underlying illness or immunosuppressive
medication and those who were pregnant were excluded. The
study was carried out between April 2008 and April 2009.
Vaccinations were administered at the Travel Clinic of the
LUMC by AR. The trial ended because the number of inclusions
Yellow fever vaccine
The live, attenuated, 17D vaccine used in this study was
manufactured on embryonated chicken eggs according to WHO
regulations and stored according to manufacturer’s guidelines. All
administered vaccines originated from the same vaccine lot
(Stamaril, Lot no B5355, Sanofi Pasteur, France). The vaccine
was administered subcutaneously in the deltoid region of the right
At the time of inclusion, data on demographic characteristics of
the participants were obtained. Blood samples for the determina-
tion of neutralizing antibodies (NA) and YF-17D viraemia were
collected before (day 0), and 3, 5, 10, 14 and 28 days after
vaccination. Participants were asked to document any injection
site and systemic adverse events after vaccination in a three-week
diary. Solicited symptoms were: erythema, pain and swelling at the
site of injection, fever and myalgia. Non-solicited symptoms could
also be reported.
Constant virus – varying serum dilution Plaque Reduction
Neutralization Test (PRNT)
The tests were carried out in 6-well plates (Corning Inc., USA)
using a slightly modified technique described originally by De
Madrid and Porterfield . Briefly, approximately 66105Vero
cells/mL were seeded per well in 6-well plates and cultured to
obtain a confluent monolayer. Coded sera were complement
inactivated at 56uC for 1 hour. Pre-vaccination sera were tested in
1:16 dilution, to which 100 plaque forming units (PFU) of 17D-YF
were added. Post vaccination sera were tested in two-fold dilutions
starting from 1:4 to 1:1024. One hundred PFU of YF-17D virus
were added to each serum dilution. All test sera were assayed in
duplicate. After 1 hour incubation on ice, the mixtures of virus
and serum were added to the Vero cell monolayers and incubated
for 1 hour at 37uC. An overlay of 26DMEM and 2% agarose was
added. After 5 days of incubation at 37uC, the overlay was
discarded and cell monolayers were stained with crystal violet.
Plaques were counted by eye by a person who had no access to the
sample code. Virus neutralization (VN) was calculated for each
serum dilution (i) with the following formula: VN(i)=100612(-
number of PFU in diluted post vaccination serum/number of PFU
in pre-vaccination serum (in a 1:16 dilution)). The serum dilution
at which log10neutralization index 0?7 (80% VN) occurred was
taken as endpoint, as this corresponds to the World Health
Organization (WHO) definition of protection . A reference
serum, obtained from the National Institute for Biological
Standards and Control (http://www.nibsc.ac.uk/) was used for
quantification of International Units per milliliter (IU/ml). In our
hands a 0.7 log10 plaque reduction in 1:10 diluted serum
corresponds to a titer of 0.5 IU/ml [95%CI 0.3–0.8 IU/ml]
. Similar values have been found by others . Geometrical
mean titers (GMT) were compared between the two groups.
Reverse Transcriptase-Polymerase Chain Reaction (RT-
Viral RNA was isolated from 200 ml plasma using a MagNa
Pure LC Total Nucleic Acid Isolation Kit (Roche Molecular
Diagnostics, Penzberg, Germany). cDNA was synthesized with
10 ml elute (200 ml total) in a professional ThermoCycler
(Biometra, Germany), and quantitative reverse transcription-
PCR (qRT-PCR) of YFV RNA was performed in a BioRad i-
cycler IQTMreal-time PCR detection system (BioRad, Veenen-
daal, The Netherlands). The following YFV specific primers and
probe were used :
YFV-P (probe) FAM-ATCGTTGAGCGATTAGCAG-BHQ
FAM (6-carboxyfluorescein) was used as 59-reporter dye and
BHQ (Black Hole Quencher) as 39-quencher dye. In order to
quantify YFV RNA, log10dilutions of in vitro transcribed RNA
standards were included as standard curves. RNA virus levels were
calculated with standard curves from Cycle threshold (Ct) values to
compare viraemia in both groups quantitatively, and were
expressed as IU/ml.
Power calculations were based on an expected 80% virus
neutralization of 95% in the control group and 66% in the elderly
group at day 14, based on previous observations at the Travel
Clinic (unpublished data). With an a of 0?05 and b of 0?2, 26
participants per group were needed to confirm a significant
difference under these assumed conditions. To take into account a
possible attrition rate of 15%, 30 participants were included per
Yellow Fever Vaccination of Elderly
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group. We analyzed the between group difference in GMT over
the four time points (day 5, 10, 14, 28) using a mixed linear model.
This model takes into account that each subject had repeated
measurements of the antibody titer over time. More specifically, a
unique identification number for each subject was entered as a
random effect in the model and separate variables for all time
points and for the groups (elderly versus young) were entered as
fixed effects. Antibody titers below the detection threshold were
assigned an arbitrary value of 0.05 IU/ml, which is twofold lower
than the lowest detectable titer (i.e. 0.2 IU/ml). Where appropri-
ate, Chi-square tests were used, and Wilcoxon’s test for non-
parametrical distributed numerical data. Statistical analysis was
performed using a computer-assisted software package (SPSS
version 16.0, SPSS Inc., Chicago, IL).
We enrolled 60 participants, none of whom withdrew
prematurely. In 2 elderly participants, 17D-YF neutralizing
antibodies were already present at day 0. In retrospect, these
participants remembered that they were vaccinated against yellow
fever many years ago. These two individuals were excluded from
further analysis. In both groups 70% were female and 30% had
visited flavivirus endemic countries in the past. The median age of
the younger participants was 21 years (interquartile range 20–22.5)
and of the elderly was 66 years (interquartile range 65–69).
Although we invited persons of 18 to 40 years of age for the
control group, the oldest participant in this group was 28 years old.
Therefore the control group is defined as age 18–30 years. We
recorded the incidence of previous travel to countries that are
endemic for flaviviruses because past infections with other
flaviviruses can cause cross-neutralization in the YF PRNT.
Neutralizing antibody response
At day 3 and 5 after vaccination, no neutralizing antibodies
were found in any of the participants. Ten days after vaccination
seroprotection was attained by 77% (23/30) of the young
participants and by 50% (14/28) of the elderly participants
(p=0.03, Chi-square test) (figure 1). The average GMT taken over
the four time points after vaccination was higher in the group of
young participants compared with the group of elderly partici-
pants. The average difference in GMT was +1.7 IU/ml (95% CI
1.2–2.4, p=0.007). At day 10 the difference was +2.9 IU/ml (95%
CI 1.8–4.7, p=0.00004) and at day 14 +1.8 IU/ml (95% CI 1.1–
2.9, p=0.02). At day 28 the difference was no longer statistically
significant(+1.5 IU/ml, 95%CI
participants in the elderly group had a higher antibody response
10 days after vaccination (female vs. male 0.04 IU/ml (95% CI
0.01–0.15) vs. 0.002 IU/ml (95%CI 0.0005–0.01), p=0.03). Such
a difference between men and women was not seen in the group of
0.9–2.4, p=0.12). Female
Yellow fever vaccine virus RNA
YF-17D viraemia was measured by qRT-PCR at day 0, 3, 5, 10
and 14 (table 1). Viraemia was detected more often in elderly (24/
28, 86%) than in young participants (18/30, 60%) (p=0.04, Chi-
square test). In addition, the elderly had higher viraemia levels
detectable for longer periods and two had detectable viraemia at
day 10, compared with none of the younger participants (table 1).
Participants reported the occurrence and duration of adverse
events after yellow fever vaccination in a 3-week diary (table 2). In
younger participants vaccination evoked erythema at the site of
inoculation more frequently and for a longer period than in the
elderly participants. In both groups, viraemia peaked at day 5. In
the group of elderly participants the mean viraemia level at day 5
was higher in those who experienced a systemic adverse event
(fever and/or myalgia) than in those who did not (viraemia level
31.3 versus 11.5 IU/ml, 95% CI for the difference 0.4–40.0 IU/
ml), p=0.05). In the group of young participants mean viraemia
levels did not differ significantly between those who did experience
Figure 1. Neutralizing antibody response against YF-17D in young and elderly participants. Reverse cumulative distribution curves of
yellow fever neutralizing antibody titers at 5, 10, 14 and 28 days after vaccination in 30 young and 28 elderly participants. Antibody titers were
determined with Plaque Reduction Neutralization Tests and reflect the serum dilution at which 80% of virus was neutralized.
Yellow Fever Vaccination of Elderly
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a systemic adverse event and those who did not (viraemia level 6.1
versus 3.9 IU/ml respectively).
The main finding of this study was that after primary
vaccination with 17D YF vaccine, elderly persons ($60 years)
were slower to develop an antibody response and had higher
viraemia levels than younger persons. Only half of the elderly
vaccinees had protective antibody levels 10 days after vaccination
compared with over three quarters of younger vaccinees. In
addition, GMT of neutralizing antibodies were significantly lower
at 10 and 14 days after vaccination. The difference was less
pronounced and no longer statistically significant 28 days after
vaccination. Besides showing higher levels of viraemia in elderly
subjects, our data also suggest that the duration of viraemia is
prolonged in these subjects as two elderly participants and none of
the younger participants had detectable viraemia at day 10.
These results provide insight into the etiology of the increased
susceptibility to YEL-AVD after yellow fever vaccination in old
age. Immunosenescence leading to an impaired ability to clear the
vaccine virus has been put forth as a possible explanation for the
increased risk of YEL-AVD in elderly people . However, in a
retrospective study of two large 17D vaccine trials involving 4,532
subjects, neutralizing antibody responses at 30 days after
vaccination were equivalent in younger and elderly subjects.
Due to the retrospective nature of that study, early responses (i.e.
,30 days after vaccination) could not be compared and were
assumed to be equal in both groups. Our results show that this
assumption needs to be modified, as we show that elderly
vaccinees are slower to develop an antibody response than
younger vaccinees. This cannot entirely explain higher age as a
risk factor for YEL-AVD, as viraemia levels peak at day 5, before
the development of neutralizing antibodies. The innate immune
response is probably also an important factor influencing viral
replication after vaccination, as suggested by Silva and colleagues
. We think that the higher viraemia levels in elderly subjects
may be due to a weaker innate immune response. Such a
hampered innate immune response together with a slower
humoral response could allow the YF-17D virus to replicate more
efficiently and for a longer period of time increasing the chance of
YEL-AVD. In this respect it is interesting to note that the
incidence of adverse events at the injection site was lower in elderly
than in younger subjects. If reactions at the injection site are the
result of immune activation, observing less injection site adverse
events in elderly subjects could reflect a weaker or slower innate
immune response in elderly persons. Similar observations were
Table 1. YF-17D viraemia measured by qRT-PCR in the elderly
group compared to young participants.
YF-17D viraemiaYoung N=30 Elderly N=28p-value
0 (0)0 (0)-
Day 3 Number
6 (20) 11 (39)0.1
IU/ml (95% CI) 1.4 (0.9–1.9)2.9 (2.1–4.4)0.04
Day 5 Number
16 (53) 23 (82)0.02
IU/ml (95% CI) 4.8 (0–10.7)20.8 (10.2–31.5) 0.07
Day 10 Number
0 (0)2 (7)0.2
IU/ml (95% CI)- 1.00 (0.8–1.2)-
0 (0) 0 (0)-
1 time point positive (%)14 (78) 12 (50)0.02
2 sequential time points
4 (22) 12 (50)
YF-17D RNA virus levels were calculated with standard curves from Cycle
threshold (Ct) values and were expressed as IU/ml. Comparison of number of
participants positive for viraemia was calculated by Fisher’s Exact test.
Comparison of quantitative viraemia (only of participants who had measurable
viraemia) was calculated with Student’s t-test. IU=International Units, 95%
CI=95% Confidence Interval.
Table 2. Solicited adverse events after primary and booster YF-17D vaccination.
Adverse event (AE) Young N=30 Elderly N=28 p-value
Injection site AEAny Yes (%)9 (30)4 (14)0.15
Days to onset (range)0 (0-2) 0.5 (0-6)0.6
ErythemaYes (%)8 (27) 2 (7)0.05
Days duration (range) 2.5 (1-8) 2 (1-3)0.4
SwellingYes (%) 3 (10)1 (4) 0.3
Days duration (range) 2 (1-5)2 (-) 1.0
PainYes (%) 3 (10)2 (7)0.7
Days duration (range)1 (1-3)2 (2-2) 0.5
Systemic AE AnyYes (%) 12 (40) 8 (29)0.4
Days to onset (range) 0.5 (0-4)5 (1-6)0.002
Myalgia Yes (%) 12 (40)6 (21)0.4
Days to onset (range)1 (0-6) 5 (1-6)0.12
FeverYes (%)3 (10) 4 (14) 0.6
Days to onset (range)0 (0-4) 5 (5-6) 0.03
Safety of vaccination expressed in various parameters. Numbers of days are medians. Fever was defined as self-measured temperature above 38 degrees Celsius. P-
values based on Chi-square test and Wilcoxon’s test. AE=Adverse event.
Yellow Fever Vaccination of Elderly
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made in an earlier study of yellow fever vaccination in elderly
Beside immunosenescence in elderly subjects, other factors
contributing to YEL-AVD have been postulated. For example, it
has previously been suggested that the vaccine virus reverts or
mutates to a more virulent form during replication in a vaccinated
individual, but extensive genetic analyses of the viral strains
extracted from patients with YEL-AVD do not provide evidence
to support this hypothesis . The possibility of host genetic
susceptibility for developing YEL-AVD seems more plausible.
Pulendran and colleagues found a heterozygous CCR5D32
mutation in a patient who suffered from YEL-AVD . Since
the prevalence of heterozygosity of the CCR5D32 mutation in the
general population is 15%  and the occurrence of YEL-AVD
among yellow fever vaccinees is significantly less [13,14],
additional host factors (e.g. immunosenescence) must also play a
role in the development of YEL-AVD . On the other hand,
milder forms of YEL-AVD might occur more frequently, but
might not be severe enough to be published, thus introducing
publication bias. Supportive of the hypothesis of genetic
susceptibility, other recently discovered genetic host factors,
including complement protein C1qB and eukaryotic translation
initiation factor 2 alpha kinase 4- (an orchestrator of the integrated
stress response) predicted YF-17D CD8+T cell responses with up
to 90% accuracy and a B-cell growth factor, TNFRS17, predicted
the neutralizing antibody response with up to 100% accuracy .
Although occurrence of YEL-AVD is very rare, fear of this
adverse event could reduce utilization of yellow fever vaccine. An
‘‘International Laboratory Network for Yellow Fever Vaccine-
Associated Adverse Events’’ has been established in 2008, to
complement the USA and the European Yellow Fever Vaccine
Safety Working Groups . Its goal is to determine the
pathogenesis of severe adverse events following yellow fever
vaccination through systematic and coordinated laboratory
evaluation of reported cases. A greater understanding of the
pathogenesis of YEL-AVD may lead to new approaches to prevent
this serious complication. One strategy may be to inject less
vaccine virus in a more immunostimulant manner (e.g. intrader-
mally) . Alternatively, inactivated YF-17D vaccine could be
used to prime the immune response which can be boosted later
with live attenuated YF-17D. This strategy has been successfully
used in mice, hamsters and cynomolgous monkeys , and more
recently Monath en co-workers have demonstrated an adequate
antibody response against yellow fever following inactivated yellow
fever vaccine .
The findings of our study can have the following practical
implication: in travelers of 60 years and older, it would be prudent
to vaccinate against yellow fever at least 14 days instead of 10 days
before departure to guarantee that all vaccinees have obtained
protective antibody levels.
Conceived and designed the experiments: AHR SAJ JTD PJB LGV.
Performed the experiments: AHR DS AWV XJ KD MG. Analyzed the
data: AHR DS. Contributed reagents/materials/analysis tools: SAJ XJ PJB
KD MG. Wrote the paper: AHR DS LGV. Revised manuscript: JTD.
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Yellow Fever Vaccination of Elderly
PLoS ONE | www.plosone.org6 December 2011 | Volume 6 | Issue 12 | e27753