Influenza continues to cause significant mortality globally and possesses substantial pandemic potential. Assessing pandemic risk requires a clear understanding of existing population immunity. Leveraging a unique large-scale cohort of human sera, we evaluated total and neutralising antibody-mediated immunity to multiple haemagglutinin (HA) proteins, including those from subtypes with high pandemic potential.

Our analysis reveals that population immunity is heterogeneous, with distinct age-dependent differences in responses to H5, H7, and H9 avian influenza subtypes. These shifts align with historical circulation patterns of seasonal H1N1 and H3N2 human viruses. Notably, H7 viruses are primarily neutralised through head domain epitopes, while H5 viruses are targeted mainly via stem epitopes, although in both instances some neutralisation occurred via receptor binding site-adjacent epitopes. Furthermore, H7 responses were dominated by non-glycan-targeted IgG2 antibodies, whereas H5 responses were primarily IgG1-mediated.

These findings highlight varying levels of susceptibility to influenza across the population, supporting vaccination approaches informed by exposure history.