Seroprotective Antibodies to 2011 Variant Influenza A(H3N2v) and Seasonal Influenza A(H3N2) among Three Age Groups of US Department of Defense Service Members

Background In 2011, a new variant of influenza A(H3N2) emerged that contained a recombination of genes from swine H3N2 viruses and the matrix (M) gene of influenza A(H1N1)pdm09 virus. New combinations and variants of pre-existing influenza viruses are worrisome if there is low or nonexistent immunity in a population, which increases chances for an outbreak or pandemic. Methods Sera collected in 2011 were obtained from US Department of Defense service members in three age groups: 19–21 years, 32–33 years, and 47–48 years. Pre- and post-vaccination samples were available for the youngest age group, and postvaccination samples for the two older groups. Specimens were tested using microneutralization assays for antibody titers against H3N2v (A/Indiana/10/2011) and seasonal H3N2 virus (A/Perth/16/2009). Results The youngest age group had significantly (p<0.05) higher geometric mean titers for H3N2v with 165 (95% confidence interval [CI]: 105–225) compared with the two older groups, aged 32–33 and 47–48 years, who had geometric mean titers of 68 (95% CI: 55–82) and 46 (95% CI: 24–65), respectively. Similarly, the youngest age group also had the highest geometric mean titers for seasonal H3N2. In the youngest age group, the proportion of patients who seroconverted after vaccination was 12% for H3N2v and 27% for seasonal H3N2. Discussion Our results were similar to previous studies that found highest seroprotection among young adults and decreasing titers among older adults. The proportion of 19- to 21-year-olds who seroconverted after seasonal vaccination was low and similar to previous findings. Improving our understanding of H3N2v immunity among different age groups in the United States can help inform vaccination plans if H3N2v becomes more transmissible in the future.

The Principal Investigator submitted a completion report for a protocol that was previously classified as minimal risk. The objective of this research project was to determine the level of serologic immunity to the 2011 H3N2v virus among 3 age groups born roughly 15 years apart and to test the hypothesis that older individuals have greater levels of immunity. Sera samples from DoD service members in 3 distinct age ranges were tested for immunity to the 2011 H3N2v virus. Fifty sera samples of recruits born in 1992-93 held at NHRC were systematically selected for testing and were supplemented with de-identified Armed Forces Health Surveillance Center (AFHSC) Department of Defense Sera Repository (DODSR) specimens from service members born in 1982-83 and 1972-73. The samples obtained from AFHSC were matched to the 1992-93 birth year data set by sex and geographic location and analyzed using microneutralization assays.
The results obtained are similar to previous studies which have found the highest sero-protection among young adults and decreasing titers among older adults. The proportion of 19-21 year olds who seroconverted after seasonal vaccination was low and similar to previous findings. Improving the understanding of H3N2v immunity among different age groups can help inform vaccination plans if I-I3N2v becomes more transmissible in the future.
I recommend forwarding the completion report to DON HRPP.

OBJECTIVE
The objective of this study was to determine the level of serologic immunity to the 2011 H3N2v virus among 3 age groups born roughly 15 years apart and to test the hypothesis that older individuals have lower levels of immunity.

APPROACH
Sera collected in 2011 were obtained from DoD Service Members in three age groups: 19-21 year olds (n=49), 32-33 year olds (n=50), and 47-48 year olds (n50). Pre-and post-vaccination samples were available for the youngest age group, and only post-vaccination for the older 2 age groups. Specimens were tested using microneutralization (MN) assays for antibody titers against H3N2v (A/Indiana/10/201 I) and seasonal H3N2 virus (A/Perth/l6/2009). A serologic MN titer of >= 1:80 was considered protective.

RESULTS/FINDINGS
The youngest age group had significantly (p<O.OS) higher geometric mean titers for H3N2v with 165 (95% CI: 105, 225), compared to the two oldest age groups, 32-33 and 47-48 year olds, who had geometric mean titers of 68 (95% CI: 55, 82) and 46 (95% Cl: 24, 65), respectively. Similarly, the youngest age group also had the highest geometric mean titers for seasonal H3N2. In the youngest age group, the proportion that seroconverted after vaccination was 12% for H3N2v and 27% for seasonal H3N2.

ADVERSE EVENTS
Not applicable. The samples used in this study were previously collected for other purposes.

CONCLUSIONS
Our results were similar to previous studies which found highest seroprotection amongst young adults and decreasing titers among older adults. The proportion of 19-21 year olds who seroconverted after seasonal vaccination was low and similar to previous findings. Improving our understanding of H3N2v immunity among different age groups in the United States can help inform vaccination plans if H3N2v becomes more transmissible in the future.