BACKGROUND: Influenza vaccination may provide a "back-boost" to antibodies against previously encountered strains. If the back-boost effect is common, this could allow more aggressive vaccine updates, as emerging variants would be expected to both elicit de-novo responses and boost pre-existing responses against recently circulating strains. Here we used the emergence of an antigenically novel A(H3N2) strain to determine whether an antigenically updated vaccine boosted antibodies against historical strains.

METHODS: We performed hemagglutination-inhibition (HI) assays on pre- and post-vaccination sera from 124 children 5-17?years old who received 2015-2016 inactivated influenza vaccine, containing an antigenically updated A(H3N2) strain. We evaluated the mean fold increase in HI titer against both the 2015-2016 vaccine strain and representative strains from two prior antigenic clusters. Factors associated with post-vaccination titers against historical strains were evaluated using linear regression, adjusting for baseline titer.

RESULTS: Geometric mean titers against each antigen examined increased significantly after vaccination (P?
CONCLUSIONS: A vaccine containing an antigenically novel A(H3N2) strain modestly boosted antibody responses against historical influenza strains in children.