Influenza A(H3N2) Subclade K During the 2025–26 Season and Differential Humoral Responses in Vaccinated Individuals Versus Vaccinated Convalescents in Israel

Background/Objectives: During the 2025–26 influenza season, the antigenically distinct A(H3N2) subclade K emerged and spread globally, raising significant concerns regarding seasonal vaccine effectiveness. This study aimed to map antigenic and structural divergence of this novel strain compared to the 2025/26 A(H3N2) vaccine component. Furthermore, we investigated how this vaccine mismatch impacts functional humoral immunity by directly comparing antibody neutralization profiles between individuals with vaccine-induced protection and those with infection-acquired immunity.