Seasonal Influenza Vaccination Is the Strongest Correlate of Cross-Reactive Antibody Responses in Migratory Bird Handlers

Avian species are reservoirs of influenza A viruses and could harbor viruses with significant pandemic potential. We examined the antibody and cellular immune responses to influenza A viruses in field or laboratory workers with a spectrum of occupational exposure to avian species for evidence of zoonotic infections. We measured the seroprevalence and T cell responses among 95 individuals with various types and degrees of prior field or laboratory occupational exposure to wild North American avian species using whole blood samples collected in 2010. Plasma samples were tested using endpoint enzyme-linked immunosorbent assay (ELISA) and hemagglutination (HA) inhibition (HAI) assays to subtypes H3, H4, H5, H6, H7, H8, and H12 proteins. Detectable antibodies were found against influenza HA antigens in 77% of individuals, while 65% of individuals tested had measurable T cell responses (gamma interferon [IFN-γ] enzyme-linked immunosorbent spot assay [ELISPOT]) to multiple HA antigens of avian origin. To begin defining the observed antibody specificities, Spearman rank correlation analysis showed that ELISA responses, which measure both head- and stalk-binding antibodies, do not predict HAI reactivities, which measure primarily head-binding antibodies. This result suggests that ELISA titers can report cross-reactivity based on the levels of non-head-binding responses. However, the strongest positive correlate of HA-specific ELISA antibody titers was receipt of seasonal influenza virus vaccination. Occupational exposure was largely uncorrelated with serological measures, with the exception of individuals exposed to poultry, who had higher levels of H7-specific antibodies than non-poultry-exposed individuals. While the cohort had antibody and T cell reactivity to a broad range of influenza viruses, only occupational exposure to poultry was associated with a significant difference in antibody levels to a specific subtype (H7). There was no evidence that T cell assays provided greater specificity for the detection of zoonotic infection. However, influenza vaccination appears to promote cross-reactive antibodies and may provide enhanced protection to novel influenza viruses.