Early influenza virus characterisation and vaccine effectiveness in England in autumn 2025, a period dominated by influenza A(H3N2) subclade K
Freja Kirsebom, Catherine Thompson, Tiina Talts, Beatrix Kele, Heather Whitaker, Nick Andrews, Nurin Abdul Aziz, Christopher Rawlinson, Rebecca E Green, Catherine Quinot, Nicholas Gardner, E. F. Waller, Alex Allen, Conall Watson, Suzanna LR McDonald, Maria Zambon, Richard Pebody, Mary Ramsay, Katja Höschler, Anika Singanayagam, Jamie Lopez Bernal
Abstract
Influenza A(H3N2) subclade K (J.2.4.1) has dominated the 2025/26 season start in England. Post-infection ferret antisera raised against northern hemisphere 2025/26 vaccine strains showed reduced reactivity to subclade K viruses in England, aligning with World Health Organization reports. Nevertheless, early post-vaccination, vaccine effectiveness against influenza-related emergency department attendances and hospital admissions remained within typical ranges, at 72-75% in children and adolescents (< 18 years) and 32-39% in adults. Hence, vaccination remains effective against clinical disease caused by influenza A(H3N2) viruses.