Abstract
We assessed haemagglutination inhibition (HAI) antibody titres against 2025 influenza A(H3N2) J.2 vaccine reference viruses and the novel subclade K (J.2.4.1) in age-stratified Hong Kong sera (collected May-July 2025), vaccine recipients and sera from virologically confirmed subclade J.2.2 or K infections. In contrast to mono-specific ferret sera, human sera with high J.2 HAI titres cross-reacted strongly with subclade K. Recombinant haemagglutinin- or egg-based standard dose J.2 A(H3N2) vaccines boosted geometric mean titres to K virus 5.6-fold and 2.6-fold, respectively.