Influenza vaccine effectiveness from nine studies during drifted A(H3N2) subclade K predominance, Europe, September 2025 to January 2026.

The World Health Organization (WHO) recommendations for the 2025/26 northern hemisphere influenza vaccination for influenza virus type A included an A/Victoria/4897/2022 (H1N1)pdm09-like and an A/Croatia/10136RV/2023 A(H3N2)-like virus for egg-based vaccines and an A/Wisconsin/67/2022 (H1N1)pdm09-like and an A/District of Columbia/27/2023 (H3N2)-like virus for cell-based vaccines [1]. Although the influenza A(H3N2) vaccine component differs from previous seasons, the vaccine recommendations for the influenza A(H1N1)pdm09 vaccine component have remained unchanged since the 2023/24 season [2,3]. The influenza season started early in some European countries, with influenza A viruses predominating between September 2025 and January 2026. Most subtyped viruses were influenza A(H3N2), among which subclade K was dominant [4]. This subclade is highly drifted from the influenza A(H3N2) virus included in the vaccine strain (subclade J.2), suggesting potential for immune escape [5,6]. However, early influenza vaccine effectiveness (VE) estimates in November and December 2025 indicated VE against influenza A(H3N2) of ≥ 50% in individuals aged younger than 65 years [7,8].