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

Avian influenza A(H9N2) viruses have been endemic in poultry for many years across Asia, the Middle East, and Northern and Western Africa and have been detected periodically in the Americas and Europe. Since their first detection in 1966, H9N2 viruses have evolved into multiple lineages and exhibit a high propensity for reassortment, contributing internal genes to zoonotic viruses such as H5N1, H7N9 and H10N8. Human infections with H9N2 viruses have been reported since 1998, with most cases linked to poultry exposure and typically resulting in mild or asymptomatic disease but severe cases and deaths have been observed as well. Despite these observations, population-level immunity to H9N2 remains poorly defined, particularly in regions without endemic exposure to the virus. Here, we assessed cross-reactive antibody responses to H9N2 viruses using a panel of recently collected human sera from the general population of a large metropolitan city in North America (New York City).