Antibodies against egg- and cell-grown influenza A(H3N2) viruses in adults hospitalized during the 2017-2018 season
BackgroundThe 2017-2018 US influenza season was severe with low vaccine effectiveness. Circulating A(H3N2) viruses from multiple genetic groups were antigenically similar to cell-grown vaccine strains. However, most influenza vaccines are egg-propagated.\n\nMethodsSerum was collected shortly after illness onset from 15 PCR confirmed A(H3N2) infected cases and 15 uninfected (controls) hospitalized adults enrolled in an influenza vaccine effectiveness study.\n\nGeometric mean titers against egg- and cell-grown A/Hong Kong/4801/2014 A(H3N2) vaccine strains and representative circulating viruses (including A/Washington/16/2017) were determined by microneutralization (MN) assays. Independent effects of strain-specific titers on susceptibility were estimated by logistic regression.\n\nResultsMN titers against egg-A/Hong Kong were significantly higher among those who were vaccinated (MN GMT: 173 vs 41; P = 0.01). However, antibody titers to cell-grown viruses were much lower in all individuals (P>0.05) regardless of vaccination. In unadjusted models, a 2-fold increase in MN titers against egg-A/Hong Kong was not significantly protective against infection (29% reduction; p=0.09), but a similar increase in cell-A/Washington titer (3C.2a2) was protective (60% reduction; p=0.02). A similar increase in egg-A/Hong Kong titer was not significantly associated with odds of infection when adjusting for MN titers against A/Washington (15% reduction; P=0.61). A 54% reduction of odds of infection was observed with a 2-fold increase in A/Washington (not significant; P=0.07), adjusted for egg-A/Hong Kong titer.\n\nConclusionAlthough individuals vaccinated in 2017-2018 had high antibody titers against the egg-adapted vaccine strain, antibody responses to cell-grown circulating viruses may not be sufficient to provide protection, likely due to egg-adaptation in the vaccine.