Immunogenicity and Cross-Reactivity of 2009–2010
Inactivated Seasonal Influenza Vaccine in US Adults and
Hang Xie1*, Xianghong Jing1., Xing Li1., Zhengshi Lin1., Ewan Plant1., Olga Zoueva1., Hong Yang2,
1Laboratory of Respiratory Viral Diseases, Division of Viral Products, Office of Vaccine Research and Review, United States Food and Drug Administration, Bethesda,
Maryland, United States of America, 2Office of Biostatistics and Epidemiology, Center for Biologics Evaluation and Research, United States Food and Drug Administration,
Bethesda, Maryland, United States of America
The campaign of 2009–2010 Northern Hemisphere seasonal vaccination was concurrent with the 2009 H1N1 pandemic.
Using a hemagglutination inhibition (HAI) assay, we evaluated the immunogenicity and cross-reactivity of 2009–2010
inactivated trivalent influenza vaccine (TIV) in US adult and elderly populations. Vaccination of TIV resulted in a robust boost
on the antibody response of all subjects to seasonal A/Brisbane/59/2007 (H1N1) and A/Uruguay/716/2007 (H3N2) with over
70% of recipients reaching a seroprotective titer of 40. B/Brisbane/60/2008 was the least immunogenic among the three
seasonal vaccine strains with ,30% of TIV recipients reaching a seroprotective titer of 40. TIV vaccination also induced a
moderate boost on the pandemic specific antibody responses. Twenty-four percent of adults and 36% of elderly reached a
seroprotective HAI titer of 40 or more against pandemic A/South Carolina/18/2009 (H1N1) after receiving TIV compared to
4% and 7% at the beginning of vaccination, respectively. In addition, 22% of adults and 34% of elderly showed an increase
of 4-fold or more in A/South Carolina/18/2009 specific HAI titers after TIV vaccination. The pandemic specific cross-reactive
antibodies strongly correlated with the post-vaccination HAI titers against the seasonal H3N2 vaccine strain in all subjects.
Citation: Xie H, Jing X, Li X, Lin Z, Plant E, et al. (2011) Immunogenicity and Cross-Reactivity of 2009–2010 Inactivated Seasonal Influenza Vaccine in US Adults and
Elderly. PLoS ONE 6(1): e16650. doi:10.1371/journal.pone.0016650
Editor: Man-Seong Park, Hallym University, Republic of Korea
Received October 18, 2010; Accepted December 22, 2010; Published January 31, 2011
This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public
domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.
Funding: This study was supported by Center for Biologics Evaluation and Research/Food and Drug Administration institutional research funds. The funders had
no role in 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: (Hang.Xie@fda.hhs.gov (HX); Zhiping.Ye@fda.hhs.gov (ZY)
. These authors contributed equally to this work.
The newly emerged 2009 pandemic H1N1 viruses have the
hemagglutinin (HA) gene derived from the classical swine lineage,
and are genetically and antigenically distinguished from recently
circulating seasonal H1N1 influenza viruses . The majority of
the population had no prior exposure to the 2009 pandemic H1N1
viruses and thus had little pre-existing immunity against these
viruses except those over the age of 60 years . Despite the
dominance of 2009 pandemic H1N1 viruses in the Northern
Hemisphere, sporadic infections caused by seasonal influenza
viruses were also reported. Hence, WHO and CDC recommended
that the public seek both seasonal and pandemic vaccines for the
2009–2010 flu season. Thanks to extensive media coverage of the
2009 pandemic and heightened public awareness of the potential
risk of influenza, a quite percentage of people have followed the
recommendation and taken the 2009–2010 seasonal vaccine
before the pandemic monovalent vaccine became widely available.
However, the effectiveness of this vaccination strategy was unclear,
especially with regard to potential impact on prevention of 2009
H1N1 pandemic. In this study, we evaluated the immunogenicity
of 2009–2010 Northern Hemisphere inactivated trivalent influen-
za vaccine (TIV) and its effects on the development of cross-
reactive antibody response to the current pandemic influenza as
measured by hemagglutination inhibition (HAI) assay.
Immunogenicity of 2009–2010 seasonal TIV
There were no significant differences in the baseline geometric
mean of HAI titers (GMTs #20) among all the subage groups
against each of the three current seasonal vaccine strains (A/
Brisbane/59/2007 (H1N1), A/Uruguay/716/2007 (H3N2), and
B/Brisbane/60/2008) at the beginning of enrollment (Figure 1A,
1B and 1C). Administration of 2009–2010 seasonal TIV induced a
robust antibody response in adults (20.1–64.8 years old) and
elderly (65.4–88.2 years old) toward the type A vaccine strains
(Figure 1A and 1B). Overall, .70% of all subjects had an HAI
titer of 40 against both H1 and H3 seasonal strains after receiving
TIV and $65% of them showed a 4-fold or more increase in the
post-vaccination titers (Figure 1D and 1E). An HAI titer of 40 has
been suggested as a seroprotective measure associated with at least
50% reduction in the risk of influenza infection or diseases in
humans [3–5]. On the contrary, both adult and elderly groups
responded significantly less robustly to B/Brisbane/60/2008 than
to the other two type A vaccine strains after TIV vaccination
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(Figure 1C and 1F). Fewer than 35% of the TIV recipients
reached a seroprotective titer of 40 or more toward B/Brisbane/
60/2008. Only 38% of adults and 19% of elderly showed a $4-
fold rise in B/Brisbane/60/2008 specific HAI titers after TIV
vaccination (Figure 1F).
Cross-reactive HAI responses before and after 2009–2010
seasonal TIV vaccination
The campaign of 2009–2010 seasonal TIV vaccination
coincided with the prevalence of 2009 H1N1 pandemic. Hence,
we were interested in how the current seasonal TIV vaccination
impacted the cross-reactive antibodies against 2009 pandemic
H1N1 viruses including A/California/07/2009 (the vaccine strain
for 2009 pandemic monovalent vaccine) and its four recent
variants (A/Iraq/8529/2009, A/England/195/2009, A/Ontario/
RV3226/2009 and A/South Carolina/18/2009). At the begin-
ning of enrollment, fewer than 10% of the participants of all ages
had detectable pre-existing antibodies against 2009 pandemic
H1N1 viruses (Figure 2A, 2B, 2C, 2D, and 2E).But 17–50% of
elderly at 80 years or older showed a baseline HAI titer $40
against pandemic A/California/07/2009, A/Iraq/8529/2009
RV3226/2009 and A/South Carolina/18/2009 (Figure 2F, 2G,
2H, 2I and 2J).
Figure 1. Hemagglutination inhibition (HAI) titers against 2009–2010 seasonal vaccine strains. Serum samples collected from US healthy
volunteers vaccinated with 2009–2010 inactivated unadjuvanted trivalent influenza vaccine (TIV) were tested by HAI assay using 0.5% turkey
erythrocytes. The geometric mean titers (GMTs), the seroprotection rates (the proportion of subjects having an HAI titer $40) before and 21 days
after TIV administration, and the seroconversion rates (the proportion of subjects having a $4-fold increase in HAI titers) were plotted according to
the age distribution of vaccinees for A/Brisbane/59/2007 (H1N1) (A and D), A/Uruguay/716/2007 (H3N2) (B and E), and B/Brisbane/60/2008 (C and F),
respectively. The 95% CI for individual HAI GMTs are shown as error bars. The dotted lines indicate HAI titer of 40.
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TIV Immunogenicity and Cross-Reactivity
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Surprisingly, both adult and elderly groups simultaneously
showed an increased antibody response to pandemic A/Califor-
nia/07/2009 and A/South Carolina/18/2009 after receiving
2009–2010 seasonal TIV, and at a less extent to the other three
pandemic variants including A/Iraq/8529/2009, A/Ontario/
RV3226/2009 and A/England/195/2009 (Figure 2A, 2B, 2C,
2D, and 2E). The most notable rise occurred in the post-
vaccination antibody response to A/South Carolina/18/2009
(H1N1), a recently isolated pandemic H1N1 variant (Figure 2D
and 2I). Twenty-four percent of adults and 36% of elderly reached
a seroprotective titer of 40 against A/South Carolina/18/2009
(H1N1) after TIV vaccination compared to 4% and 7% at the
enrollment respectively (Figure 2D). Additionally, 22% of adults
and 34% of elderly showed a$4-fold increase in A/South
Carolina/18/2009 specific HAI titers after receiving TIV
(Figure 2I). In particular, 42% of the subjects born before 1930
showed increased antibody response to A/South Carolina/18/
2009 by a factor of 4 or more following TIV vaccination
(Figure 2I). The cross-reactive antibody responses to A/Califor-
nia/07/2009 (H1N1) and A/South Carolina/18/2009 (H1N1)
after TIV vaccination were comparable to that induced by B/
Brisbane/60/2008 (Figure 1F, 2A and 2D). A correlation analysis
indicated that post-vaccination HAI titers against pandemic A/
California/07/2009 and A/South Carolina/18/2009 strongly
correlated with those against seasonal A/Uruguay/716/2007
(H3N2) in all TIV recipients (p,0.0001, Figure 3A and 3B).
The same correlations were observed between the post-vaccina-
tion HAI titers against the other three pandemic viruses and
seasonal A/Uruguay/716/2007 (H3N2) (data not shown).
The effectiveness of seasonal vaccines largely relies on how well
selected vaccine strains represent circulating influenza viruses in
the environment. Antigenically poorly matched vaccine strains
could lead to significantly reduced protection in vaccinated
population as shown in the 2007–2008 flu season . Hence, it
is necessary to continue annual evaluation of influenza vaccine
strains as currently led by WHO through its collaborative centers
over the world. As one of the WHO collaborative centers, each
year our laboratory will prescreen human vaccinated serum
samples collected from US seasonal clinical trials and select those
having post-vaccination titers $40 against all three vaccine strains
for distribution to other collaborative centers for further
evaluation. In January of 2010, 24 pairs (before and after seasonal
vaccination) each of selected human clinical samples representing
adult and elderly populations in US were distributed for annual
vaccine strain evaluation. Because of the predominance of 2009
pandemic H1N1 viruses in circulations, however, the antibody
responses to the seasonal H1 vaccine strain were not considered
. By the same token, the cross-reactivity of the 2009–2010
seasonal TIV toward the pandemic H1N1 viruses was not
But it does not necessarily imply that the 2009–2010 seasonal
TIV played no role in protecting against 2009 H1N1 pandemic,
since a suboptimal vaccine could still provide some protection
against influenza-related hospitalization, especially in persons with
high risk medical conditions [8,9]. In addition, a good percentage
of US population has received 2009–2010 seasonal TIV before the
pandemic monovalent vaccine became widely available. Thus, it
was of value to evaluate the impact of seasonal vaccination on the
development of cross-reactive antibodies against pandemic viruses.
With the expanded sample sizes including all the subjects enrolled
in US seasonal trials, we showed in the present study that both
adult and elderly groups had increased antibody responses to
pandemic vaccine strain A/California/07/2009 (H1N1) and a
newly isolated variant A/South Carolina/18/2009 (H1N1) after
receipt of 2009–2010 TIV. This is unlikely due to prior infection of
Figure 3. Correlation of antibody titers specific for seasonal vaccine strains and pandemic viruses after seasonal vaccination. Serum
samples were collected on 21 days after administration of 2009–2010 inactivated unadjuvanted trivalent influenza vaccine (TIV) in US healthy adults
and elderly. An HAI assay was performed using 0.5% turkey erythrocytes. A correlation analysis between seasonal strain specific HAI titers and
pandemic strain specific HAI titers was performed using nonparametric Spearman’s r test by JMP Version 7. The scatterplots of HAI titers are shown
in the presence of 95% bivariate normal density ellipses (indicating the distribution of 95% of individual data points plotted) and corresponding
p values. A, A/California/07/2009 (H1N1) vs A/Uruguay/716/2007 (H3N2); B, A/South Carolina/18/2009 (H1N1) vs A/Uruguay/716/2007 (H3N2).
Figure 2. Hemagglutination inhibition (HAI) titers against 2009 pandemic H1N1 viruses. Serum samples collected from US healthy
volunteers vaccinated with 2009–2010 inactivated unadjuvanted trivalent influenza vaccine (TIV) were tested for cross-reactivity against 2009
pandemic H1N1 viruses by HAI assay using 0.5% turkey erythrocytes. The pandemic influenza specific seroprotection rates (the proportion of subjects
having an HAI titer $40) before and 21 days after TIV administration and the seroconversion rates (the proportion of subjects having a $4-fold rise in
HAI titers) were plotted according to the age distribution of vaccinees for A/California/07/2009 (H1N1) (A and F), A/Iraq/8529/2009 (H1N1) (B and G),
A/Ontario/RV3226/2009 (H1N1) (C and H), A/South Carolina/18/2009 (H1N1) (D and I) and A/England/195/2009 (H1N1) (E and J), respectively.
* indicates the corresponding seroconversion rate in panel F, G, H, I, and J.
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2009 pandemic since the majority of the subjects born after 1930
had a very low baseline titer (,10) against these viruses. On the
other side, concurrent pandemic infection during the seasonal TIV
vaccination might not be the cause either, otherwise the subjects at
80 years or older would not respond to A/South Carolina/18/
2009, since 50% of them already had seroprotective antibody titers
against the pandemic vaccine strain A/California/07/2009 before
TIV vaccination. Hence, the moderate boost on pandemic specific
antibody responses observed in the current study was more likely
resulted from 2009–2010 TIV vaccination. Lee et al. (2010) also
reported that the 2009–2010 Southern Hemisphere seasonal TIV
induced a certain degree of cross-reactive antibodies in healthy
young military recruits against a local 2009 H1N1 pandemic
isolate . This suggested 2009–2010 TIV vaccination regardless
of Northern or Southern Hemisphere formulation could provide
some cross-immunity against 2009 H1N1 pandemic at least in
adults. This was new because Hancock et al. (2009) reported that
seasonal vaccinations prior to 2009 induced little or no cross-
reactive antibodies against pandemic H1N1 viruses . Howev-
er, Plennevaux et al. (2009) also reported that the adults with
seasonal influenza vaccination during the 2004–2009 periods had
significantly higher HAI titers and seroprotection rates against
2009 pandemic than the subjects without prior seasonal
vaccinations . Using a highly sensitive pseudovirus based
influenza HA neutralization assay, Labrosse et al. (2010) found
that the subjects with 2008–2009 seasonal vaccination showed
significantly higher neutralizing titers against pandemic A/
California/04/2009 (H1N1) than the subjects without the same
seasonal vaccination . They also found a strong correlation
between the neutralizing titers against A/Brisbane/59/2009
(H1N1) and A/California/04/2009 (H1N1) . Interestingly,
we found in the present study that the post-vaccination HAI titers
against A/California/07/2009 (H1N1) and A/South Carolina/
18/2009 (H1N1) strongly correlated with their seasonal H3
specific antibody response instead seasonal H1 specific antibodies
following 2009–2010 TIV administration. Additionally, it was out
of many experts’ initial anticipation that a single 15 mg HA dose of
pandemic monovalent vaccine was found to be sufficient enough
to induce a robust antibody response and a priming-boosting
regimen was unnecessary in most of vaccinees, despite these swine-
origin H1N1 viruses were new to the public [12,14,15]. This may
be partially attributed to the priming effect of the current and prior
seasonal vaccinations, since a substantial portion of subjects
enrolled in the 2009 pandemic monovalent clinical trials had
already received the 2009–2010 seasonal vaccine .
A moderate boost on cross-reactive response to a novel
influenza virus by seasonal vaccination may not lead to a complete
protection against a pandemic, but might reduce the burden of
infections substantially in affected subjects, which is often seen in
young children receiving partial vaccination [16–18]. A surveil-
lance study on military service members stationed in the US soil
has found that prior seasonal vaccinations between 2004–2009
were positively associated with protection against clinically
apparent and laboratory-confirmed 2009 pandemic H1N1 illness
. This study along with another study led by L. Garcia-Garcia
in Mexico City also found that the cross-protective effect of
seasonal vaccination was more obvious in subjects with severe
symptoms or hospitalization than those with mild outcome .
Controversies also exist. The Canadian public petitioner D.M.
Skowronski and her team have claimed that an increased risk of
2009 pandemic H1N1 illness was present in local communities
after receipt of seasonal vaccines [21,22]. However, the bias or
confounding factors could not be ruled out as these always being
the major concern for observational studies. Interestingly, another
cohort study on hospital nurses from Ontario, Canada has
reported a possible positive effect of the 2008–2009 TIV on
reducing risk of 2009 pandemic infections though the sample size
was small . A more recent study by Cowling et al. (2010) has
found no direct link between the 2008–2009 Southern Hemi-
sphere seasonal TIV and increased risk of 2009 H1N1 pandemic
infection in children at ages of 6–15 years from Hong Kong .
In addition to the pandemic specific cross-reactive antibody
responses elicited by 2009–2010 TIV, our data also indicated that
B/Brisbane/60/2008 was significantly less immunogenic than the
other two type A vaccine components that, only 33% of adult and
elderly subjects reached a seroprotective titer of 40 or more against
B/Brisbane/60/2008 after TIV vaccination. The low immuno-
genicity of B/Brisbane/60/2008 was likely due to that it was a
new strain unlike A/Brisbane/59/2007 (H1N1) and A/Uruguay/
716/2007 (H3N2) (an A/Brisbane/10/2007-like virus) that have
been vaccine components since 2008–2009 season. However,
repeated immunization may overcome this problem since B/
Brisbane/60/2008 remains as a vaccine component for the 2010–
2011 influenza season in the Northern Hemisphere.
Nevertheless, ours and others’ studies suggest that annual
seasonal vaccination play an important role in protecting the
public not only against seasonal flu but also a pandemic. In the
United States, the seasonal influenza vaccination coverage jumped
significantly during the 2009–2010 flu season, especially in
children and adults aged 18–49 years without high risk medical
condition , which should be greatly attributed to the joint
campaign by governments and media. Continuous seasonal
vaccination on individuals and expanding vaccination coverage
in the community could potentially reduce disease burden from
routinely circulating or newly emerging influenza viruses in future.
Materials and Methods
All the human pre- and post-vaccinated serum samples involved
in the present study were analyzed anonymously at CBER/FDA.
Vaccinated human serum samples
Paired serum panels (pre-seasonal vaccination and post-seasonal
vaccination) from 2009–2010 TIV vaccinated healthy human
volunteers were kindly provided by Dr. Mark Blatter (Primary
Physicians Research, Inc., Jefferson Hills, PA), Dr. Michael J. Noss
(Radiant research, Cincinnati, OH), and Dr. Rex Biedenbender
(Eastern Virginia Medical School, Norfolk, VA) respectively in
support of WHO consultation on the composition of influenza
vaccine for the Northern Hemisphere. A written informed consent
was provided to all volunteers at the beginning of the enrollment
by each physician according to the guidelines of the corresponding
institutional review board. Unless otherwise indicated, 120 adults
(20.1–64.8 years old) and 59 elderly (65.4–88.2 years old) from US
were given a single dose (15 mg HA/strain/dose) of 2009–2010
unadjuvanted TIV intramuscularly (Table 1). The TIV product
mentioned above was formulated according to Northern Hemi-
sphere seasonal vaccine composition containing A/Brisbane/59/
2007 (H1N1), A/Uruguay/716/2007 (H3N2), and B/Brisbane/
60/2008) as recommended by WHO. Sera were collected at the
time of enrollment and 21 days after TIV vaccination.
Hemagglutination inhibition (HAI) assay
The 2009–2010 seasonal TIV vaccine strains A/Brisbane/59/
2007 (H1N1), A/Uruguay/716/2007 (H3N2), and B/Brisbane/
60/2008 and the 2009 H1N1 pandemic viruses A/California/07/
2009, A/Iraq/8529/2009, A/Ontario/RV3226/2009, A/South
TIV Immunogenicity and Cross-Reactivity
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Carolina/18/2009, and A/England/195/2009 were propagated
in 9-day-old specific pathogen-free embryonated chicken eggs.
The immunogenicity of 2009–2010 seasonal TIV was evaluated
by HAI assay using 4 HA units of viruses and 0.5% turkey
erythrocytes according to the procedures described previously
[26,27]. Sera were pre-treated with receptor-destroying enzyme
(RDE, Denka-Seiken, Tokyo, Japan) at 37uC for 18 h followed by
heat-inactivation at 56uC for another 30 min. RDE-treated sera
were then serially two-fold diluted starting from 1:10 dilution prior
to HAI assay. A titer of 5 was assigned if negative HAI reactions
occurred at $1:10.
Data analysis and statistics
All subjects including adults and elderly were mixed together
and grouped according to the ages with 10-year-intervals with
demographic characteristics shown in Table 1. Three immunoge-
nicity end points were chosen for the final data analysis: the
geometric mean of HAI titers (GMTs); the seroprotection rate (the
proportion of subjects with HAI titers $40); and the seroconver-
sion rate (the proportion of subjects with a $4-fold rise in HAI
titers). A correlation analysis between seasonal strain specific HAI
titers and pandemic specific HAI titers after 2009–2010 TIV
administration was performed using nonparametric Spearman’s
r test by JMP (Version 7). A p value of ,0.05 was considered
We sincerely appreciate Dr. Mark Blatter (Primary Physicians Research,
Inc., Jefferson Hills, PA), Dr. Michael J. Noss (Radiant research,
Cincinnati, OH), and Dr. Rex Biedenbender (Eastern Virginia Medical
School, Norfolk, VA) for providing vaccinated human sera. We are grateful
to Dr. Xiyan Xu, Influenza Division, Centers for Diseases Control and
Prevention for providing seasonal and pandemic influenza viruses. We also
thank Drs. Carol Weiss and Vladimir Lugovtsev, Division of Viral
Products, Office of Vaccine Research and Review, CBER/FDA for critical
review of this manuscript.
Performed the statistical analysis: HY HX. Conceived and designed the
experiments: HX ZY. Performed the experiments: HX XJ XL ZL EP OZ
ZY. Analyzed the data: HX. Wrote the paper: HX. Performed the
statistical analysis: HY HX.
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Table 1. Demographic characteristics of the subjects vaccinated with 2009–2010 seasonal inactivated trivalent vaccine.
Characteristic 20–29.9 Year30–39.9 Year 40–49.9 Year50–59.9 Year 60–69.9 Year70–79.9 Year80–89.9 Year
No. of subjects25 17 311742 3512
Median age 24.6 years36.7 years45.2 years 55.2 years 63.5 years73.4 years83.4 years
NOTE. The 2009–2010 Northern Hemisphere inactivated trivalent influenza vaccine (TIV) contained 15 mg of hemagglutinin of each of the following strains: A/Brisbane/
59/2007 (H1N1), A/Uruguay/716/2007 (H3N2), and B/Brisbane/60/2008.
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