The higher (30 µg) dosage did not appear to improve the frequency of seroprotective responses or seroconversions in the 2 older age groups (Table 2). However, there was a trend towards better responses to the higher titer vaccine within the youngest stratum at day 42, but these differences were not statistically significant (seroconversion P = 0.07 and seroprotection P = 0.07; Table 2). A post hoc evaluation suggested that seroconversions were not more likely to occur among children who developed local reactogenicity after either dose of vaccine.
The impact of dosage, gender and prior seasonal influenza immunization on geometric mean titer (GMT) at each time point was explored using stratum-specific linear models (controlling for baseline HAI titers and clinical site). There were no consistent effects of gender or dosage on GMT 21 days after either dose. Prior receipt of 2008–2009 seasonal influenza vaccination significantly reduced GMT for the middle age stratum at all visits, reaching statistical significance at day 8 after dose 1 (P = 0.03) and at 21 days after dose 2 (P = 0.01), and significantly lowered GMT within the oldest age stratum after dose 2 (P = 0.01 for both days 8 and 21).
This multicenter, randomized, double-blind trial demonstrates that monovalent, inactivated influenza A(H1N1)pdm09 vaccine was well-tolerated by children aged 6 months to 17 years when given as 1 or 2 doses of either 15 or 30 µg (22–25 or 47 µg by single radio immunodiffusion, respectively). Although a single 15 µg dose elicited seroprotective titers in 91% of participants aged 10–17 years, children <10 years required 2 doses to meet this endpoint, and the proportion who achieved a 1:40 titer was significantly lower (78–82%). Similar observations were noted after the 30 µg dose.
Preliminary findings from this study informed national policy regarding the use of influenza A(H1N1)pdm09 vaccine in children and now provide a unique opportunity to examine age-related responses to inactivated influenza vaccine that may be helpful in formulating future recommendations for both pandemic and seasonal influenza vaccination of children. Of particular interest is the immunogenicity among the youngest children who experience high attack rates and morbidity from influenza but historically produce inferior responses compared with immunologically primed older children and adults.24,25 Initial reports during the pandemic unexpectedly indicated that a 15 μg dose of inactivated influenza A(H1N1)pdm09 vaccines elicited seroprotective responses in many infants and young children, raising the possibility that a single dose might be sufficient.26–28 Postvaccination HAI titers ≥1:40 were reported in 50–92% of children 6–35 months of age26,27 and in 64–93% of children 3–11 years of age,26–28 whereas in our trial, only 20% and 47% of similarly aged children achieved this endpoint. Of note, Plennevaux et al26 evaluated a prevalidation lot of Sanofi Pasteur vaccine comparable with that tested in our trial, and despite comparable baseline titers, fewer subjects in our trial seroconverted or developed seroprotective responses. Differences in assay performance, vaccine components and exposure of participants to wild-type infection or other immunologically priming events, such as prior seasonal vaccine receipt, might explain some of the variability among trials.29 Nonetheless, our results are consistent with post-licensure assessments which suggest that despite a close match with circulating virus strain, a single dose was not significantly effective at preventing medically attended influenza illness among children ≤9–13 years of age,30–32 but provided significant protection in adolescents and young adults.31–33 Most studies of seasonal influenza vaccine in previously unvaccinated young children have reported similar results.13,34,35
These findings suggest that more immunogenic vaccination strategies are needed to ensure that infants and young children are protected against influenza, particularly during pandemics when timing of vaccine availability may not permit delivery of 2 spaced doses. There are data to suggest that live attenuated influenza vaccines are more efficacious and effective than inactivated vaccines even after 1 dose,30,32,36,37 but 2 doses are recommended, and in the United States, live attenuated influenza vaccines can only be given to eligible children aged ≥2 years. Oil-in-water adjuvants such as AS03 (which are licensed for children in many countries but not in the United States) had dose-sparing effects and elicited protective immunity against influenza A(H1N1)pdm09 after a single dose in children 6 months to 8 years.38–40 Likewise, a 15 μg dosage of MF59-adjuvanted TIV was significantly more protective than TIV alone among children 6–35 months and 36–71 months of age.41 Nonetheless, possible safety signals in children who received AS03-containing pandemic vaccines require further scrutiny.42 Another potential strategy for enhancing immunogenicity in young children is the use of higher doses of antigen. Whereas the full adult dose (45 µg HA) is standard for children aged 6–35 months in other countries,43,44 half that amount is recommended in the United States45 stemming from reactions such as febrile seizures associated with historic whole-cell inactivated vaccines,44,46,47 but seen uncommonly with currently licensed split virus and subunit vaccines. Our study provides evidence that inactivated influenza vaccine is safe and well-tolerated when doses of up 30 µg (47 µg by single radio immunodiffusion) are given to children aged 6–35 months. Although we saw a trend toward increased immunogenicity after the higher dosage, the differences appeared only after the second dose and did not reach statistical significance. Among Canadian infants aged 6–11 months, 2 full (45 µg) doses of TIV induced higher responses than 2 half doses for all 3 vaccine components without increasing reactogenicity, reaching statistical significance for the H3N2 and B components.48 Studies further exploring a high dose strategy in United States children are being conducted in the Vaccine and Treatment Evaluation Units.
Our trial provided a unique opportunity to examine the maximum age for which a 2-dose regimen is beneficial, as there is little information to address this issue. Two doses are recommended through the age of 8 years for seasonal vaccination, but the maximum age was increased to 9 years during the influenza A(H1N1)pdm09 pandemic. Our logistic regression model suggests that FDA performance criteria for seroprotection among immunologically naïve subjects (70% lower bound of the 2-sided 95% CI for the percent of subjects achieving an HAI antibody titer ≥1:40)23 was achieved after a single 15 µg dose around 10 years of age, thus supporting the recommended increase in age limit for pandemic vaccines. Further examination of the most appropriate age cutoff for giving 2 doses of both pandemic and seasonal influenza vaccines is warranted to ensure that all age groups are adequately protected.
To our knowledge, this is the first study that evaluated day 8 responses to influenza A(H1N1)pdm09 vaccines in children. The occurrence of “anamnestic” responses, as reflected by a rapid increase in antibody within 8 days postvaccination, indirectly demonstrates the existence of immune memory.49,50 As predicted, the proportion of children achieving HAI titers ≥1:40 on day 8 increased with age, but the magnitude of seroprotective titers in the middle (31%) and oldest (84%) strata was unexpected given the degree of divergence of the A(H1N1)pdm09 virus HA sequence from the strains that have circulated during the past 2 decades. Serologic cross-reactivity between the influenza A(H1N1)pdm09 virus and seasonal H1N1 strains that would not be considered antigenically close has been observed,51 but the mechanism and competence of these responses remains elusive. Another example of the potential for previous exposure to influenza antigen to shape the antibody response to later exposures may be the observation in our study and elsewhere that immune responses were significantly diminished in individuals who previously received 2008–2009 seasonal influenza vaccine.27,52–54
Several limitations of our study are noteworthy. Without a placebo group, we cannot assess the potential of effect of intercurrent infections with wild-type virus, which was circulating at the time of our trial, on the immune responses after vaccination. Our study was not powered to detect rare adverse events or subtle dose-related differences in immune response. Finally, caution must be exercised in extrapolating our results to programmatic use of vaccines because the lots we tested contained more HA than the licensed vaccine and because a half dose (7.5 µg) is currently recommended for children aged 6–35 months.
In conclusion, we demonstrated that inactivated influenza A(H1N1)pdm09 vaccine was well-tolerated in children aged 6 months to 17 years at dosages comparable to adult dosages of seasonal TIV. Although a single 15 µg dose induced seroprotective HAI antibody responses in nearly all children 10–17 years of age, 2 doses were needed for younger children. Considering the logistical challenges of delivering 2 doses, particularly during an influenza pandemic, vaccines that confer protective immunity to children <10 years of age after a single inoculation are desirable. Using higher dosages of vaccines or adding adjuvants to enhance immune responses warrant additional investigation.
The authors thank the children and their families who participated in this study, and the members of the study teams who contributed in many ways to the conduct of this trial including: Adetinuke (Mary) Boyd, Julia Hutter, Kimberly Rincavage Wilhelmi, Nancy Wymer, Linda Wadsworth, Ginny Cummings, Mardi Reymann and Inna Ruslanova (University of Maryland); Dwight Fortier and the members of Annapolis Pediatrics; Wayne Crowder and the members of The Pediatric Center of Frederick; Todd Callahan, Deborah Hunter, Belinda Gayle Johnson, Shanda Phillips, Wendi McDonald, Faith Brendle, Alice O’Shea, Deloris Lee, Lana Howard, Melissa Lehman, Christa Hedstrom, Deborah Myers, Kevin Booth and Roberta Winfrey (Vanderbilt University); Melissa Seybert; Terry Buford, Kirsten Weltmer, Mary Anne Jackson, Denise Bratcher, Jason Newland, Angela Myers, Douglas Swanson, Robyn Livingston, and Gina Calarco (Children’s Mercy Hospital and Clinics); Catherine Bull, Morty Cohen, Elle Ficken, Diane Kinnunen, Susan Jacob, Kirsten Lacombe, Jenna Lane and Aimee M Verrall (University of Washington); members of the Seattle Children’s Hospital Clinical Research Center; Ballard Pediatric, Mercer Island Pediatric, Northwest Pediatrics, North Seattle Pediatrics and Woodinville Pediatric Clinics; Lynn Harrington, Beth Patterson, Kathlene Chmielewski, Lori Hendrickson, Luis Ballon, Kathryn Lattimore, Liz Schmidt Susan Doyle and Amanda Anderson (Duke University); and Barbara Taggart, Valerie Johnson, Candi Looney, F. Owen Griffin and Shixiong Li (Southern Research Institute). The authors thank our colleagues at the NIAID/DMID: Wendy Buchanan, Richard Gorman, Robert Johnson, Tena Knudsen, Linda Lambert, Robin Mason, Suzanne Murray and Shy Shorer. The authors also wish to acknowledge and thank colleagues at BARDA/DHHS and Sanofi Pasteur for providing the vaccines.
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