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. 2025 Nov;30(46):2500854.
doi: 10.2807/1560-7917.ES.2025.30.46.2500854.

Early influenza virus characterisation and vaccine effectiveness in England in autumn 2025, a period dominated by influenza A(H3N2) subclade K

Freja Cm Kirsebom  1 , Catherine Thompson  2 , Tiina Talts  2 , Beatrix Kele  2 , Heather J Whitaker  3 , Nick Andrews  1 , Nurin Abdul Aziz  1 , Christopher Rawlinson  1 , Rebecca E Green  1 , Catherine Quinot  1 , Nicholas Gardner  1 , Elizabeth Waller  1 , Alex Allen  1 , Conall H Watson  1   4 , Suzanna Lr McDonald  1 , Maria Zambon  2 , Richard Pebody  4   5 , Mary Ramsay  6   7 , Katja Hoschler  2 , Anika Singanayagam  2   4   8 , Jamie Lopez Bernal  1   4   8
Affiliations

Early influenza virus characterisation and vaccine effectiveness in England in autumn 2025, a period dominated by influenza A(H3N2) subclade K

Freja Cm Kirsebom et al. Euro Surveill. 2025 Nov.

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.

Keywords: A(H3N2); Influenza; J.2.4.1; subclade K; test-negative design; vaccine effectiveness.

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Conflict of interest statement

Conflict of interest: The Immunisations and Vaccine Preventable Diseases division at UKHSA has undertaken post-marketing surveillance and regulatory analyses requested by influenza vaccine manufacturers for which cost-recovery charges have been made. No other conflicts of interest have been declared.

Figures

Forty-one influenza A(H3N2) viruses of determined subclade, circulating in England between March and October 2025, were characterised by haemagglutination inhibition assay using post-infection ferret antisera respectively raised against current and recent vaccine strains. From August, 10 subclade K viruses were detected, all showing over 32-fold reactivity reduction with antisera raised against the egg-propagated A/Croatia/10136RV/2023 vaccine strain and at least eightfold reduction with antisera against cell-propagated A/District of Columbia/27/2023. Of eight K subclade viruses tested with antisera against A/England/189/2025, four reacted within a fourfold reduction of the homologous titre. Of note, A/England/189/2025 is similar to the southern hemisphere 2026 vaccine strain, A/Sydney/1359/2024-like virus.
Figure 1
Influenza A(H3N2) viruses antigenically characterised by haemagglutination inhibition (HAI) assay, England, March–October 2025 (n = 41 viruses)
The figure shows influenza vaccine effectiveness against influenza A overall and against influenza A subtype A(H3N2). Effectiveness of the vaccine is measured by its ability to prevent clinical disease that causes people to go to hospital and test positive for influenza. For children and teenagers up to 17 years old, the vaccine effectiveness against preventing clinical disease due to influenza A or A(H3N2) ranges between 72 and 75%. For adults in their mid-60s and older, this ranges between 32 and 39%. For younger adults, vaccine effectiveness is between 32 and 33% for influenza A, but between 60 and 66% for A(H3N2).
Figure 2
Vaccine effectiveness against emergency department attendance and hospital admission for children and adolescents aged 2 to 17 years, adults aged 18 to 64 years and adults aged ≥ 65 years, England, 29 September to 2 November 2025 (n = 28,789 cases and controls)
See this image and copyright information in PMC

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