Methods: Children aged 5-17 years in Marshfield, WI were recruited in fall 2015 to provide serum before and 28 days after vaccination with inactivated influenza vaccine (IIV). Participants were assigned into strata based on prior (2014-15) season vaccination status: IIV (IIV-IIV group), live attenuated vaccine (LAIV-IIV), or unvaccinated (U-IIV). Serum samples were tested by hemagglutination inhibition (HI) against the A/California/7/2009 (H1N1)pdm09-like reference strain. Endpoints were seroconversion (defined as ≥4 fold titer rise), seroprotection (post vaccine titer ≥1:40), and mean fold rise (MFR). Illness surveillance was conducted and respiratory swabs were tested for influenza during each episode of respiratory illness.
Results: Of the 126 children in the study, 77 (61%) were in the IIV-IIV group, 13 (10%) in the LAIV-IIV group, and 36 (29%) in the U-IIV group. Pre-vaccination geometric mean titer (GMT) was 169 (95% CI 133-213) for IIV-IIV, 64 (95% CI 24-172) for LAIV-IIV, and 41 (95% CI 26-64) for U-IIV (p<0.0001). Seroprotection was 99%, 100%, and 92% for IIV-IIV, LAIV-IIV, and U-IIV, respectively (p=0.1). Percent seroconversion was 16% for IIV-IIV, 62% for LAIV-IIV, and 89% for U-IIV (p=<0.0001). U-IIV had the highest MFR (17.2, 95% CI 11.7-25.4), which was significantly higher than the mean fold rise for LAIV-IIV (8.3, 95% CI 3.9-17.8) (p=0.01) and IIV-IIV (2.0, 95% CI 1.7-2.2) (p<0.0001). MFR for LAIV-IIV was also significantly higher than IIV-IIV (p<0.0001). There were two PCR positive vaccine failures, both prior season IIV recipients, who had post-vaccination titers of 1:57 and 1:320.
Conclusion: Prior season unvaccinated individuals and LAIV recipients experienced a significantly greater antibody rise to 2015-16 IIV relative to prior season IIV recipients. However, all groups had high post-vaccination titers. There was no serologic evidence to support variation in vaccine effectiveness against H1N1pdm09 due to prior season vaccine patterns.
B. Flannery, None
S. Spencer, None
E. Belongia, MedImmune: Investigator , Research support
Novavax: Investigator , Research support