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Immunogenicity and Safety of a Trivalent Inactivated Influenza Vaccine in Children 6 Months to 17 Years of Age, Previously Vaccinated with an AS03-Adjuvanted A(H1N1)Pdm09 Vaccine

Two Open-label, Randomized Trials

Vesikari, Timo MD, PhD*; Richardus, Jan Hendrik MD, PhD; Berglund, Johan MD, PhD; Korhonen, Tiina MD*; Flodmark, Carl-Erik MD, PhD§; Lindstrand, Ann MD, MPH; Silfverdal, Sven Arne MD, PhD, MPH; Bambure, Vinod MSc**; Caplanusi, Adrian MD, PhD††; Dieussaert, Ilse IR‡‡; Roy-Ghanta, Sumita MD§§; Vaughn, David W. MD, MPH‡‡

The Pediatric Infectious Disease Journal: July 2015 - Volume 34 - Issue 7 - p 774–782
doi: 10.1097/INF.0000000000000709
Vaccine Reports
Free
SDC

Background: During the influenza pandemic 2009–2010, an AS03- adjuvanted A(H1N1)pdm09 vaccine was used extensively in children 6 months of age and older, and during the 2010–2011 influenza season, the A(H1N1)pdm09 strain was included in the seasonal trivalent inactivated influenza vaccine (TIV) without adjuvant. We evaluated the immunogenicity and safety of TIV in children previously vaccinated with the AS03-adjuvanted A(H1N1)pdm09 vaccine.

Methods: Healthy children were randomized (1:1) to receive TIV or a control vaccine. Children were aged 6 months to 9 years (n = 154) and adolescents 10–17 years (n = 77) when they received AS03-adjuvanted A(H1N1)pdm09 vaccine at least 6 months before study enrolment. Hemagglutination inhibition (HI) and neutralizing antibody responses against the A(H1N1)pdm09 strain were evaluated before (day 0) and at day 28 and month 6 after study vaccination. Reactogenicity was assessed during the 7 day postvaccination period, and safety was assessed for 6 months.

Results: At day 0, >93.9% of all children had HI titers ≥1:40 for the A(H1N1)pdm09 strain, which increased to 100% at both day 28 and month 6 in the TIV group. Between days 0 and 28, HI antibody geometric mean titers against A(H1N1)pdm09 increased by 9-fold and 4-fold in children 6 months to 9 years of age and 10–17 years of age, respectively.

Conclusion: AS03-adjuvanted A(H1N1)pdm09 vaccine-induced robust immune responses in children that persisted into the next season, yet were still boosted by TIV containing A(H1N1)pdm09. The reactogenicity and safety profile of TIV did not appear compromised by prior receipt of AS03-adjuvanted A(H1N1)pdm09 vaccine.

From the *Vaccine Research Center, University of Tampere Medical School, Tampere, Finland; Department of Infectious Disease Control, Municipal Public Health Service (GGD) Rotterdam-Rijnmond, Rotterdam, The Netherlands; School of Health Science, Blekinge Institute of Technology, Karlskrona, Sweden; §Childhood Obesity Unit, Skåne University Hospital, Malmö, Sweden; Department of Public Health Sciences, Karolinska Institutet, Stockholm, Sweden; Department of Clinical Sciences, Umeå University, Umeå, Sweden; **Quanticate, Bangalore, India; ††GSK Vaccines, Wavre, Belgium; ‡‡GSK Vaccines, Rixensart, Belgium; and §§GSK Vaccines, King of Prussia, PA

Accepted for publication December 15, 2014.

GlaxoSmithKline Biologicals SA was involved in all stages of the study conduct and analysis (ClinicalTrials.gov Identifiers: NCT01196026 and NCT01190215). GlaxoSmithKline Biologicals SA also took in charge all costs associated with the development and the publishing of the present manuscript. All authors had full access to the data. The corresponding author had final responsibility to submit for publication. This study was sponsored by GlaxoSmithKline Biologicals SA. V.B., A.C., I.D., S.R. and D.W.V. are/were employees of GSK group of companies. A.C., I.D., S.R. and D.W.V. report owning stock in the company. T.V. received payment and travel expenses for advisory board memberships from GSK, Merck, Sanofi Pasteur MSD and Pfizer. J.H.R. received funding from GSK to perform the trial. GSK has funded other vaccine trials at his institution, outside the submitted work. J.B. and J.H.R. declare receiving payment for travel expenses from GSK at the time of the study. S.S. declares payment from GSK for advisory board membership, lectures and travel expenses outside the submitted work. C.F., T.K. and A.L. have nothing to disclose.

Trademark statement: Fluarix, Havrix and Pandemrix are trademarks of GSK group of companies.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website (www.pidj.com).

Address for correspondence: Timo Vesikari, MD, PhD, Vaccine Research Center, University of Tampere Medical School, Biokatu 10, 33520 Tampere, Finland. E-mail: timo.vesikari@uta.fi.

The 2009 swine-origin A(H1N1)pdm09 pandemic caused substantial global morbidity and mortality. Infection and related hospitalizations among children were much higher than that observed in adult populations.1,2 As well as being highly susceptible to contracting influenza, children are important for viral transmission, and as such, after the emergence of the A(H1N1)pdm09 strain, the World Health Organization recommended that children should be a priority group for vaccination.3 One vaccine that was licensed for use in children ≥6 months of age was an inactivated split virion A(H1N1)pdm09 vaccine, adjuvanted with AS03, an oil-in-water emulsion containing α-tocopherol, squalene and polysorbate 80 (PandemrixTM; GSK Vaccines, Rixensart, Belgium); this vaccine was administered to an estimated 9.5 million children during the pandemic in 2009–2010.4,5

The development and approval of AS03-adjuvanted A(H1N1)pdm09 vaccine was based on experience with prototype H5N1 influenza vaccine, which had been shown to provide robust and cross-reactive, immune responses in children and adults.6,7 Before the start of vaccination, there were limited immunogenicity data for the A(H1N1)pdm09 vaccine strain in children, yet it soon emerged that the A(H1N1)pdm09 vaccine antigen was highly immunogenic in children 6 months to 17 years of age.8,9 Although the recommended vaccination schedule was initially 2 doses based on experience with AS03-adjuvanted H5N1 prototype vaccines, 1 dose of AS03-adjuvanted A(H1N1)pdm09 vaccine was found to elicit strong antibody responses even in immunologically naïve children, and a single dose was found to be effective against laboratory-confirmed infection in children and young adults.10

During the pandemic in 2009–2010, in addition to AS03-adjuvanted vaccines containing 1.9–3.75 μg of A(H1N1)pdm09 hemagglutinin antigen (HA), nonadjuvanted vaccines containing 15 μg of HA were also found to be highly immunogenic in children.11–13 As such, the World Health Organization and the Advisory Committee on Immunization Practices recommended a transition from monovalent vaccine against A(H1N1)pdm09 to the inclusion of 15 μg of A(H1N1)pdm09 HA without adjuvant in the seasonal trivalent inactivated influenza vaccine (TIV) for the 2010–2011 influenza season in the Northern Hemisphere, given that the pandemic strain was predicted to be the dominant circulating A/H1N1 virus.14

In this article, we present the results of 2 phase IV studies that were conducted to assess the immunogenicity, reactogenicity and safety of TIV containing the A(H1N1)pdm09 strain in children 6 months to 9 years of age and 10–17 years of age who had previously received 2 half doses or 1 full dose, respectively, of AS03-adjuvanted A(H1N1)pdm09 vaccine. The main aim of the study was to evaluate the persistence and boostability of the A(H1N1)pdm09 immune response. The study also evaluated the effect of the A(H1N1)pdm09 immune response on immunogenicity against the other TIV antigens and the safety of TIV in children who had previously received AS03-adjuvanted vaccine.

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METHODS

Design and Participants

This report details phase IV, open-label, randomized, multicenter studies that assessed the immunogenicity, reactogenicity and safety of TIV in children 6 months to 17 years of age at the time they received AS03-adjuvanted A(H1N1)pdm09 vaccine at least 6 months before enrolment. One study enrolled children in Sweden (6 centers) and the Netherlands (1 center) between September and November 2010, and the other enrolled children at a single center in Finland between October and December 2010.

Eligible children had previously received AS03-adjuvanted A(H1N1)pdm09 vaccine: children 6 months to 9 years of age had received 2 half doses, of which the last dose was given at least 6 months before enrolment, and children 10–17 years of age received 1 full dose given at least 6 months before enrolment. Children were healthy and without acute or chronic pulmonary, cardiovascular, renal or hepatic functional abnormality. Children were excluded if they had received any registered or investigational drug or vaccine within 30 days of the study vaccines or administration of any vaccine was planned during 30 days before and after the study vaccines. Other exclusion criteria were acute fever (oral temperature ≥37.5°C) at enrolment, hypersensitivity to influenza vaccine or its components, history of seizures, immunosuppression, receipt of immunoglobulins or blood products within 3 months preceding vaccination, receipt of an A(H1N1)pdm09 vaccine other than AS03-adjuvanted A(H1N1)pdm09 vaccine and receipt of the 2010–2011 seasonal influenza vaccine.

The studies were conducted in accordance with Good Clinical Practice guidelines, the Declaration of Helsinki and local regulations. All study-related documents were approved by the appropriate ethics committees. Parents provided written informed consent before commencement of the study, and adolescents gave their assent. Clinical trial identifiers: NCT01196026 and NCT01190215.

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Vaccines and Schedule

All the vaccines in the study were manufactured by GSK Vaccines. The TIV (FluarixTM) was an inactivated split virion trivalent influenza vaccine containing the recommended strains for the 2010/2011 influenza season in the Northern Hemisphere; a 0.5 mL dose contained 15 μg HA of each strain: A(H1N1)pdm09, A/Victoria/210/2009 [H3N2]-like and B/Brisbane/60/2008-like. The control vaccine was a monovalent inactivated hepatitis A vaccine (Havrix JuniorTM and HavrixTM), given as a 0.5 mL dose containing 720 Elisa units of hepatitis A antigen, or a 1.0 mL dose containing 1440 Elisa units of hepatitis A antigen. The AS03-A(H1N1)pdm09 vaccine (Pandemrix) given before the study was a 0.5 mL dose containing 3.75 μg of A(H1N1)pdm09 HA with thimerosal (5 μg per dose) as a preservative for the multidose vials and adjuvanted with AS03A containing α-tocopherol (11.86 mg), squalene (10.69 mg) and polysorbate (4.86 mg); or half the volume (0.25 mL) providing half the amount of each of the vaccine components, with the adjuvant dose designated as AS03B.

A randomization list was generated by the sponsor, and treatment allocation at study centers was performed via an internet-based system; in both studies children were randomized 1:1 to receive TIV or control vaccine, and in the study of children 6 months to 9 years of age, the groups were balanced for age at initial vaccination with AS03-adjuvanted A(H1N1)pdm09 vaccine: 6–11 months, 12–35 months and 3–9 years.

Children 6 months to 9 years of age had previously received 2 half doses of AS03-A(H1N1)pdm09 given 21 days apart, and children 10–17 years of age had received 1 full dose of AS03-A(H1N1)pdm09. All children allocated to TIV received 1 dose on day 0, and unprimed children received a second dose of TIV on day 28 (unprimed defined as children aged <9 years who had not previously received a seasonal TIV). Safety and reactogenicity were assessed by a safety review team for 7 days after each dose of TIV; the safety team provided a recommendation to proceed with the administration of the next dose of vaccine for each unprimed subject throughout each vaccination phase. Children allocated to the control group received control vaccine on days 0 and 180 according to the summary of product characteristics recommendations. The second dose of control vaccine was given outside of the study setting. Vaccines were administered in the deltoid region of the nondominant arm by nonblinded personnel.

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Objectives

The primary objective was to evaluate hemagglutination inhibition (HI) antibody responses against the A(H1N1)pdm09 strain 28 days after 1 dose of TIV in children 6 months to 9 years of age who previously received two 1.9 μg doses of A(H1N1)pdm09 HA formulated with AS03B and in children 10–17 years of age who previously received one 3.75 μg dose of A(H1N1)pdm09 HA formulated with AS03A. Secondary objectives were to assess HI responses against each of the TIV strains at day 28 and month 6 in children 6 months to 9 years of age and 10–17 years of age and by age substrata (6–11 months, 12–35 months and 3–9 years); to assess neutralizing antibody responses against each of the TIV strains at day 28 and month 6 in children 6 months to 9 years of age and 10–17 years of age and by age substrata (6–11 months, 12–35 months and 3–9 years); to assess HI and neutralizing antibody responses against the A(H1N1)pdm09 strain in the control vaccine group at day 0 and month 6; to assess reactogenicity during the 7-day postvaccination period after dose 1 of TIV or control; to assess adverse events (AEs), serious AEs (SAEs), potentially immune mediated diseases (pIMDs) from day 0 to month 6 in the TIV and control groups.

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Assessments

Immunogenicity

In children who received TIV, serum samples were collected before vaccination (day 0), at days 28 and 182 (month 6). In children who received control vaccine, samples were collected at day 0 and month 6. HI assays were performed at GSK Vaccines central laboratory using a validated in-house assay (cutoff = 1:10) that used chicken erythrocytes, as described previously.15 Viral microneutralization assays were performed on serum samples collected at days 0, 28 and 182 (month 6) at GSK Vaccines central laboratory using a validated in-house assay; the microneutralization assay was based on a 50% neutralization titer calculated by the Reed and Muench method and had a cutoff of 1:28, as previously described.16–18 There was a problem adapting the A/Victoria/210/2009 strain to make proper read out of the assay, so the A/H3N2 microneutralizing test was performed on A/Perth/16/2006, which is a A/Victoria-like strain.

HI antibody parameters were geometric mean titers (GMT), seroprotection rates (SPR; percentage of children with HI titers ≥1:40) and seroconversion rates (SCRs; percentage of children achieving an increase in HI titers from <1:10 to ≥1:40 or at least a 4-fold postvaccination increase in HI titer from a prevaccination titer ≥1:10). Neutralizing antibody parameters were GMTs, seropositivity rates (antibody titer ≥1:8) and vaccine response rates (VRR; percentage of children achieving an increase in neutralizing antibody titers from <1:8 to ≥1:32 or at least a 4-fold postvaccination increase in titer from a prevaccination titer ≥1:8).

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Safety and Reactogenicity

Parents used diary cards to record solicited local and general AEs for 7 days following vaccination. Local AEs were pain, redness and swelling, and general AEs were fever, irritability/fussiness, drowsiness and loss of appetite (<6 years of age) or arthralgia, fatigue, gastrointestinal, headache, myalgia, shivering, sweating and fever (≥6 years of age). Fever was defined as a temperature ≥38.0°C axillary temperature or ≥37.5°C oral temperature. The intensity of solicited symptoms was graded (0–3); grade 1 symptoms were defined as not interfering with normal activities, and grade 3 symptoms were defined as preventing normal activities (grade 3 redness and swelling: diameter >50 mm; grade 3 fever: axillary temperature >39°C).

Unsolicited AEs were recorded for 28 days after vaccination, and SAEs, medically attended AEs (MAEs) and pIMDs were recorded for 6 months postvaccination. All solicited injection site symptoms were considered vaccination-related, and investigators provided causality assessments for solicited general AEs and unsolicited AEs.

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Statistical Analyses

The target sample size was 360 children 6 months to 9 years of age (60 to receive TIV or control in each age stratum: 6–11 months, 12–35 months and 3–9 years) and 120 children 10–17 years of age. The sample size was estimated to have 20% precision in the estimation of postvaccination GMTs.

The analyses of HI antibody and neutralizing antibody responses included children who met eligibility criteria, fulfilled protocol-defined procedures and who had serological data available at a given time point: days 0 and 28 (per-protocol immunogenicity cohort) and month 6 (per-protocol immunogenicity persistence cohort). Immunogenicity analyses in the control group were performed on the per-protocol immunogenicity persistence cohort. All descriptive statistics of HI and neutralizing antibody responses were tabulated with 95% confidence intervals.

Reactogenicity and safety analyses were performed on the total vaccinated cohort including children who received ≥1 dose of vaccine and with any safety data available. The incidence of reactogenicity and safety events was tabulated with 95% confidence intervals.

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RESULTS

A total of 239 children were enrolled in the 2 studies, 162 were 6 months to 9 years of age and 77 were 10–17 years of age when they received the monovalent vaccination against A(H1N1)pdm09 (Fig. 1). The recruitment target was not fulfilled. Demographic details for the 2 age groups are shown in Table 1.

TABLE 1

TABLE 1

FIGURE 1

FIGURE 1

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Immunogenicity

A(H1N1)pdm09

HI antibody responses for A(H1N1)pdm09 by age strata are shown in Table 2. The mean interval from the last dose of AS03-adjuvanted A(H1N1)pdm09 vaccine to day 0 was 293.7 days (range 187–353 days; TIV group) for children 6 months to 9 years of age and 379.8 days (range 274–692 days; TIV group) for children 10–17 years of age. Before vaccination with TIV (day 0), 96.9% of children 6 months to 9 years of age (younger stratum) and 93.9% of children 10–17 years of age (older stratum) had HI titers ≥1:40 for the A(H1N1)pdm09 strain, and GMTs of 120.7 and 150.1, respectively. At day 28 after TIV, all children in both age groups had HI titers ≥1:40 for the A(H1N1)pdm09 strain, and GMTs increased to 1079.3 (9-fold increase) and 646.8 (4-fold increase), respectively. After TIV, SCRs at day 28 against the A(H1N1)pdm09 strain were 84.6% in the younger age stratum and 48.5% in the older age stratum. In the per-protocol immunogenicity persistence cohort, at month 6 after TIV, GMTs against the A(H1N1)pdm09 strain were 509.0 and 346.4 in the younger and older age strata, respectively.

TABLE 2

TABLE 2

At month 6 in TIV recipients, all the children in the younger and older age strata had HI titers ≥1:40 against A(H1N1)pdm09. In the control group in the per-protocol immunogenicity persistence cohort, in the younger and older age strata, GMTs against A(H1N1)pdm09 at day 0 were 154.6 and 152.7, respectively, and at month 6 were 120.7 and 131.4, respectively. In the control group, the proportions of children with HI titers ≥1:40 against A(H1N1)pdm09 at day 0 were 100% and 91.9% in the younger and older age strata, respectively, and at month 6 were 92.9% and 91.9%, respectively. Immunogenicity in the control group is shown in Table 3.

TABLE 3

TABLE 3

Neutralizing antibody responses by age strata are shown in Table 4. In the per-protocol immunogenicity cohort, at day 0 in the younger (6 months–9 years) and older (10–17 years) strata, the GMTs against A(H1N1)pdm09 were 247.3 and 119.7, respectively, and at day 28 were 4007.7 and 1512.4, respectively. In the per-protocol immunogenicity cohort, at day 0 in the younger (6 months–9 years) and older (10–17 years) strata, the GMTs against A(H1N1)pdm09 were 247.3 and 119.7, respectively, and at day 28 were 4007.7 and 1512.4, respectively. At day 28, the VRR against A(H1N1)pdm09 in the younger and older age strata was 88.9% and 87.9%, respectively. In the per-protocol immunogenicity persistence cohort, at month 6 for the older and younger strata, GMTs against A(H1N1)pdm09 were 1748.2 and 390.6, respectively, and all children were seropositive (titer ≥1:8). In the per-protocol immunogenicity persistence cohort for the control group for the younger and older age strata, all children were seropositive for A(H1N1)pdm09 at day 0, and 100% and 97.3% were seropositive, respectively, at month 6. In the younger and older age strata in the control group, GMTs against A(H1N1)pdm09 at day 0 were 329.3 and 138.3, respectively, and at month 6 were 368.3 and 115.3, respectively (Table 3).

TABLE 4

TABLE 4

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A/H3N2 and B/Brisbane

Robust HI antibody responses after TIV were observed against A/Victoria (H3N2) and B/Brisbane for both age strata. In the younger and older age strata, the SPR (HI titers ≥1:40) against A/H3N2 increased from 46.2% and 36.4% at day 0, respectively, to 100.0% in all children at day 28. In the younger and older age strata, the SPR against B/Brisbane increased from 21.5% and 39.4%, respectively, at day 0, to 96.9% and 100.0%, respectively, at day 28. The SCRs in the younger and older strata against A/H3N2 at day 28 were 98.5% and 87.9%, respectively, and against B/Brisbane were 84.6% and 84.8%, respectively. There was good antibody persistence to month 6 for A/H3N2 (GMTs of 186.8 and 160.1 in the younger and older strata, respectively) and B/Brisbane (154.1 and 242.4, respectively; Table, Supplemental Digital Content 1, http://links.lww.com/INF/C110). The proportion of children in the younger age stratum with HI titers ≥1:40 against A/H3N2 and B/Brisbane was 100% and 92.9%, respectively, and in the older stratum was 97.1% and 94.3%, respectively. Neutralizing antibody responses by age strata are shown in Table, Supplemental Digital Content 2, http://links.lww.com/INF/C111. At day 28, in the younger and older age strata, the VRR against A/H3N2 was 81.3% and 66.7%, respectively, and against B/Brisbane was 39.1% and 75.8%, respectively.

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Reactogenicity and Safety

Reactogenicity during the 7-day postvaccination period after the first dose of TIV or control vaccine is shown in Figures 2 and 3. In the TIV group, the most common solicited local AE for children 6 months to 9 years of age was injection site redness (52 of 76; 68.4%) and for children 10–17 years of age was injection site pain (35 of 38; 92.1%). Local AEs were generally less frequent in the control than in the TIV groups. In the control group, injection site pain was the most common solicited local AE for all age groups. Apart from grade 3 redness, which was reported by 11.6% of children 6 months to 9 years of age in the TIV group, grade 3 local AEs were reported at a rate of ≤5.3%.

FIGURE 2

FIGURE 2

FIGURE 3

FIGURE 3

The most frequently reported solicited general AEs for the TIV group were irritability (18 of 53; 34.0%) and loss of appetite (16 of 53; 30.2%) for children 6–35 months of age and fatigue (25 of 38; 65.8%) and headache (24 of 38; 63.2%) for children 10–17 years of age.

In children 6 months to 9 years of age during the 28-day postvaccination period after dose 1 for the TIV group, at least 1 unsolicited AE was reported by 21 of 77 (27.3%) children and for the control group by 25 of 77 (32.5%) children. For the TIV group, the most common unsolicited AEs were cough (5.2%), gastroenteritis (3.9%) and upper respiratory tract infection (3.9%), and for the control group were upper respiratory tract infection (7.8%), cough (5.2%) and nasopharyngitis (3.9%).

For children 10–17 years of age during the 28-day postvaccination period after dose 1 in the TIV group, at least 1 unsolicited AE was reported for 12 of 38 (31.6%) children and for the control group for 11 of 39 (28.2%) children. The most common unsolicited AEs for the TIV group were upper respiratory tract infection (7.9%) and neck pain (5.3%) and for the control group were upper respiratory tract infection (5.1%), headache (5.1%) and oropharyngeal pain (5.1%). For the TIV and control groups, ≤3.9% and ≤5.1% of unsolicited AEs, respectively, were grade 3 in children 10–17 years of age.

During the 6-month follow-up period in all children, 3 MAEs were reported in 3 children who received TIV (upper respiratory tract infection, head trauma and acute tonsillitis) and 1 SAE (acute tonsillitis in the 10–17 years group), which were not considered to be related to vaccination. No pIMDs were reported during the study.

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DISCUSSION

This study represented the real-life situation where children received monovalent AS03-adjuvanted A(H1N1)pdm09 vaccine during the 2009–2010 influenza pandemic followed by TIV containing the A(H1N1)pdm09 strain during the 2010–2011 influenza season. The target recruitment of 480 children was not fulfilled, and our study included 239 children, which was sufficient to provide a description of immunogenicity and reactogenicity and the opportunity to assess antibody responses in children who were primed with adjuvanted influenza vaccine and boosted with nonadjuvanted influenza vaccine in the following season. However, as the target recruitment was not met, the postvaccination GMT values are descriptive, and the immunogenicity data should be interpreted accordingly.

Our analyses suggested that in children 6 months to 17 years of age who received AS03-adjuvanted A(H1N1)pdm09 during the 2009–2010 pandemic, immune responses against the A(H1N1)pdm09 strain persisted well for over a year after vaccination; in the TIV group, after a mean interval of 293.7 days (6 months to 9 years) and 379.8 days (10–17 years) following the last dose of monovalent vaccine, the proportion of children with HI titers ≥1:40 against A(H1N1)pdm09 was 96.9% and 93.9% in the younger (6 months–9 years) and older (10–17 years) age strata, respectively. All children had HI titers of ≥1:40 against the A(H1N1)pdm09 strain 28 days and 6 months after receiving TIV during the 2010–2011 influenza season. For children who received AS03-adjuvanted A(H1N1)pdm09 vaccine followed by monovalent inactivated hepatitis A vaccine, HI antibodies against the A(H1N1)pdm09 strain elicited by the initial vaccine persisted at high levels (100% and 92.3% of children in the younger and older age strata, respectively, had HI titers of ≥1:40 against A(H1N1)pdm09 at month 6), suggesting that boosting with TIV was not necessary to maintain high antibody titers after AS03-adjuvanted A(H1N1)pdm09 vaccine.

The results of our study are consistent with other reports of antibody persistence and immunogenicity among children who received AS03-adjuvanted A(H1N1)pdm09 in 2009–2010 followed by TIV during the 2010–2011 influenza season, which showed that antibodies against A(H1N1)pdm09 persisted for 6 months after the initial vaccination, and that a single dose of TIV elicited antibody levels considered to be protective against A(H1N1)pdm09, resulting in a booster effect with potential to provide protection across 2 seasons.19,20 In the study conducted by Walker et al,20 100% of children 6 months to 3 years of age and 96.9% of children 3–12 years of age had neutralizing antibody titers ≥1:40 against the A(H1N1)pdm09 strain 1 year after vaccination, and all children had neutralizing antibody responses ≥1:40 against the pandemic strain contained in the 2010–2011 TIV. In the study conducted by Gilca et al,19 antibody persistence was observed against the A(H1N1)pdm09 strain 1 year after vaccination with AS03-adjuvanted A(H1N1)pdm09 in children 15–120 months of age, and 98.4% children had HI antibody titers ≥1:40 against the pandemic strain after the subsequent dose of TIV.

Previous studies have shown that the receipt of seasonal influenza vaccination may affect immune responses to subsequent AS03-adjuvanted A(H1N1)pdm09 vaccination in adults,10,21 but there are limited information about immunogenicity to seasonal influenza vaccination subsequent to AS03-adjuvanted A(H1N1)pdm09. In our study, 28 days after 1 dose of TIV, HI SCRs against the A(H1N1)pdm09 strain were relatively low (84.6% and 48.5% in the younger and older age stratum, respectively), compared with the A/H3N2 (98.5% and 87.9%) and B/Brisbane (84.7% and 84.8%) strains; the relatively low HI SCRs for A(H1N1)pdm09 were likely associated with the high HI antibody titers at baseline before TIV vaccination, and conversely, the high SCRs against A/H3N2 and B/Brisbane were associated with low levels of antibody at the time of vaccination. Nonetheless, despite the high baseline titers for A(H1N1)pdm09, “boostability” was still observed with TIV.

At the time of the study, the Swedish Medical Products Agency and the National Institute for Health and Welfare of Finland were investigating the clustering of new onset narcolepsy cases in temporal relation with AS03-adjuvanted A(H1N1)pdm09 vaccination.22,23 Several retrospective studies are in agreement with the early reports regarding an association between vaccination with Pandemrix during the 2009–2010 pandemic and the subsequent onset of narcolepsy in people less than 21 years of age and in adults.23–27 Further research is needed to understand the chain of events that resulted in narcolepsy and the potential roles of genetic factors,28 environmental factors29–31 or a combination of these factors.32,33 Our study did not include a large enough population to detect rare events such as narcolepsy.

The reactogenicity profile of TIV showed an increased rate of local symptoms for TIV recipients than for the control group, particularly in 10–17 years old. It should be noted that the comparator vaccine hepatitis A vaccine has a low reactogenicity. There were no pIMDs during the 6-month follow-up, the rate of SAEs was low (n = 1), and thus there were no major safety concerns identified overall.

The strengths of the study were the randomized, controlled design, and that it represented the real-life influenza pandemic vaccination situation in which an adjuvanted influenza vaccine was followed by nonadjuvanted influenza vaccine including the pandemic strain administered across 2 influenza seasons in children 6 months to 17 years of age. This prime–boost schedule provided rapid immune responses that persisted for 18 months. Further studies are needed to investigate whether priming with adjuvanted vaccine could enable flexible influenza vaccination schedules, potentially providing long-term protection and cross-reactivity against circulating strains. However, a major limitation of the study was the lower than planned sample size. A further limitation was that although children with fever ≥37.5°C at enrolment were not eligible for inclusion, previous influenza episodes were not an exclusion criterion. Indeed, the circulation of A(H1N1)pdm09 was high during the study, and natural exposure to the pandemic virus may have contributed to the observed antibody responses.

In summary, although the study sample was small, and the recruitment target was not fulfilled, our results are of interest because they suggest that 1 full dose or 2 half doses of AS03-adjuvanted A(H1N1)pdm09 vaccine elicited robust immune responses that persisted above levels that are considered to be protective for at least 6–18 months, and that A(H1N1)pdm09 HA incorporated into seasonal TIV further boosted the antibody levels. The “booster” dose of A(H1N1)pdm09 in the seasonal TIV did not appear to negatively affect immunogenicity against the other seasonal strains and did not appear to compromise safety. These findings support a prime–boost schedule using an adjuvanted influenza vaccine followed by nonadjuvanted influenza vaccine administered across 2 influenza seasons in children 6 months to 17 years of age.

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ACKNOWLEDGMENTS

The authors are grateful to the New York Medical College, New York, for providing the vaccine virus strain. The authors are indebted to the participating study volunteers, clinicians, nurses and laboratory technicians at the study sites. The authors are grateful to the principal investigators, Drs. Jan-Eric Eriksson, Skellefteå, Sweden and Leif Ekholm, Örobro, Sweden, from the study sites; to the study staff in Tampere Vaccine Research Clinic in Finland and to all teams of GSK Vaccines for their contribution to this study, especially the clinical and serological laboratory teams, Evelien Lintner (Valesta, on behalf of GSK Vaccines), Karolien Peeters (independent, on behalf of GSK Vaccines), Jan Brissinck (GSK Vaccines) and Julie De Wever (Keyrus Biopharma on behalf of GSK Vaccines) for preparation of the study protocol and related study documentation, Dorothy Slavin (Clinical Safety Representative), and Karl Walravens (Clinical Immunology Representative; both GSK Vaccines). Finally, the authors thank Annick Moon (Moon Medical Communications, UK) and Avishek Pal (GSK Vaccines) for medical writing services and Santosh Mysore and Shirin Khalili (XPE Pharma & Science, on behalf of GSK Vaccines) for editorial assistance and manuscript coordination.

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Keywords:

A(H1N1)pdm09; TIV; AS03; influenza; vaccine

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