Vaccine-Elicited Antibody Responses to Influenza A(H3N2) Subclade K

Influenza A(H3N2) subclade K (J.2.4.1) is a genetic branch of H3N2 with 11 mutations in hemagglutinin compared with the A/H3N2/Croatia/10136RV/2023 (H3N2 CR/23) vaccine strain, of which 8 mutations are on the hemagglutinin head surface (Figure, A) (eFigure 1 in Supplement 1).1 As of February 2026, influenza A viruses currently represent approximately 96.3% of circulating influenza strains in the US, with H3N2 accounting for 88.4% of influenza isolates and subclade K comprising 91.5% of H3N2 isolates.2,3 The rapid expansion of H3N2 subclade K represents a major public health concern.4,5 This study reports antibody responses to H3N2 subclade K and other influenza strains before and after influenza vaccination.