Genetic Diversity, Reassortment Patterns, and Antigenic Characterization of Two H9N2 Avian Influenza Viruses Isolated from Quails in China.

H9N2 avian influenza viruses pose a persistent zoonotic risk owing to their broad host adaptability. Between 2024 and 2025, two H9N2 isolates (ZHY1022B2 and ZHY0417A11) were recovered from quail flocks in Hebei, China. Phylogenetic analysis clustered their HA genes within the dominant B4.7.2 subclade, but diverged into two subgroups (B4.7.2.2 and B4.7.2.1). Of particular interest was ZHY0417A11, which displayed a multigenic reassortment pattern-its PB1 originated from an H3N8 virus, PA and PB2 from H3N3, NS from H10N3, and the M gene was nearly identical (99.32%) to a human H3N8 isolate, whereas the remaining HA, NA and NP segments maintained the H9N2 backbone. Despite the presence of the HA mammalian-adaptive markers (H191N, A198V and Q234L), both strains showed marked antigenic drift from the vaccine strain SS (BJ/94-like; R < 0.5), while retaining reactivity with currently circulating field strains. These findings argue for heightened vigilance in quail populations, given their role as mixing vessels, and highlight the limitations of current vaccine matching in light of ongoing H9N2 evolution.