Hemagglutination inhibition antibody titers mediate influenza vaccine efficacy against symptomatic influenza A(H1N1), A(H3N2), and B/Victoria infections.

HAI titers partially mediate influenza vaccine efficacy against influenza A(H1N1), A(H3N2), and B/Victoria. Our estimates were lower than previous studies, possibly reflecting expected heterogeneity in antigenic similarity between vaccine and circulating viruses across seasons. Data from more influenza seasons are needed to understand better the mediating effects of HAI titers on vaccine efficacy.