Influenza A(H3N2) subclade K (J.2.4.1) viruses associated with a surge at a university health clinic, Arizona, the United States, November to early December 2025

Genomic surveillance during an influenza surge between November and early December 2025 at a university health clinic in the United States identified A(H3N2) subclade K (J.2.4.1) viruses with shared haemagglutinin amino acid substitutions in antigenic sites and the receptor-binding domain. An epitope-based model indicated reduced vaccine protection (mean predicted protection 0.13). Phylodynamic analyses suggested multiple introductions with onward campus-to-community spread, highlighting that universities and other semi-closed settings can amplify transmission and aid early characterisation of emerging lineages.