Cross-reactive human antibody responses to H9N2 influenza virus, New York, United States, 2025

Understanding population immunity to emerging viruses is critical for risk assessment. We characterised antibody responses to avian influenza A(H9N2) viruses in 298 human sera from adults in New York City. We observed widespread cross-reactive H9-binding antibodies, albeit at lower levels (GMT = 371.3) than against seasonal H3 (GMT = 1,398.0), with low to undetectable haemagglutination inhibition and neutralising antibodies. In contrast, NA-binding and neuraminidase inhibition responses were moderate (GMT = 149.5 and 34.2) and comparable to those against human seasonal N2 (GMT = 259.0 and 42.2).