Abstract
BACKGROUND: The hemagglutination inhibition (HAI) assay is an established correlate of protection for the inactivated influenza vaccine. However, the proportion of vaccine-induced protection that is mediated by the post-vaccination HAI titer has not been assessed. METHODS: We used data from a randomized, placebo-controlled trial of a split-virion inactivated influenza vaccine in children aged 6-17 years. Sera were collected before and 30 days after receipt of vaccination or placebo and tested by the HAI assay against B/Brisbane/60/2008-like (B/Victoria lineage). We fitted Cox proportional hazards models to the time to laboratory-confirmed influenza B. We used causal mediation analysis to estimate the proportion of the total effect of vaccination that was mediated by higher HAI titers. RESULTS: We estimated that vaccine efficacy against confirmed B/Victoria infection was 68% (95% confidence interval, 33%, 88%), and post-vaccination HAI titers explained 57% of the effect of vaccination on protection. CONCLUSIONS: The majority of the effect of inactivated influenza vaccination in children is mediated by the increased HAI titer after vaccination; however, other components of the immune response to vaccination may also play a role in protection and should be further explored. Causal mediation analysis provides a framework to quantify the role of various mediators of protection.