Abstract Background: Gaps remain in our knowledge of the humoral correlates of protection against influenza A/H3N2, including the role of antibodies against the hemagglutinin stalk, a highly conserved but immunologically sub-dominant region. Methods: Two household studies were conducted in Managua, Nicaragua. Household contacts were tested for influenza using RT-PCR. We compared pre-existing antibody levels against full-length hemagglutinin (FLHA), HA stalk, and neuraminidase (NA) measured by ELISA, along with initial HA inhibition assay (HAI) titers, between infected and uninfected participants. Results: A total of 899 individuals participated in household activation during three A/H3N2 seasons with 329 infections occurring. A four-fold increase in initial HA stalk titers was independently associated with an 18% decrease in the risk of infection (OR=0.82, 95%CI 0.68-0.98, p=0.04). In 0-14-year-olds, anti-NA antibodies (OR=0.67, 95%CI 0.53-0.85, p<0.01) were associated with protection, but anti-HA stalk antibodies were not. In adults, anti-HA stalk antibodies were independently associated with protection (OR=0.72, 95%CI 0.54-0.95,p=0.02). Conclusions: The HA stalk is an independent correlate of protection against A/H3N2 infection, though this protection is age dependent. This supports the continued exploration of the HA stalk as a target for broadly protective influenza vaccines and suggests that the protective benefits may depend on age and influenza exposure history.
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