VIROLOGY
in 2003/4 with the emergence of the Fujian variant which caused unusually high number of deaths in children. The emergence of A(H1N1)pdm09 in
2009 was an example of an influenza pandemic arising from a zoonotic origin; which required replacement of the H1N1 component of influenza vaccines with the new pandemic strain. So far, the impact of the K variant in most NH countries has been to cause an early influenza season, with moderate impact. The final impact assessment will be made when the winter season has completed, and all the data are available on hospitalisation.
Further Reading Dunn D. Preparing for another pandemic:
RCGP Research and Surveillance Centre. Pathology in Practice. 2025 April;25(2):48-52 European Centre for Disease Prevention and Control. Early estimates of seasonal influenza vaccine effectiveness against influenza requiring medical atention at primary care level in Europe, week 41 - 49, 2025. (ECDC,2025)
www.ecdc.europa.eu/en/news- events/early-estimates-seasonal-influenza- vaccine-effectiveness-against-influenza- requiring European Centre for Disease Prevention and Control. European Respiratory Virus Surveillance Summary. (ECDC, 2026) www.
erviss.org Kirsebom FC, Thompson C, Talts T, et al. Early influenza virus characterisation and vaccine effectiveness in England in autumn 2025, a period dominated by influenza A(H3N2) subclade K. Euro Surveill. 2025;30(46):2500854. doi:10.2807/1560-7917. ES.2025.30.46.2500854 Neher RA, Huddleston J, Bedford T,
The ideal vaccine viral antigens confer
a broad breadth of immunity to multiple subclades of viruses, thus reducing the risk of a vaccine mismatch
PPi
et al. Nomenclature for Tracking of Genetic Variation of Seasonal Influenza Viruses. Influenza Other Respir Viruses. 2026;20(2):e70230. doi:10.1111/irv.70230 Sabaiduc S, Kaweski SE, Separovic L, et al. Emergence of seasonal influenza A(H3N2) variants with immune escape potential warrants enhanced molecular and epidemiological surveillance for the 2025–2026 season. J Assoc Med Microbiol Infect Dis Can. 2025;10(4):281-98. doi:10.3138/ jammi-2025-002 Separovic L, Sabaiduc S, Zhan Y, et al. Interim 2025/26 influenza vaccine effectiveness estimates with immuno- epidemiological considerations for A(H3N2) subclade K protection, Canada, January 2026. Euro Surveill. 2026;31(5):2600068. doi:10.2807/
1560-7917.ES.2026.31.5.2600068 UK Health Security Agency. UK National COVID and Influenza Surveillance Reports, 2025-2026. (
Gov.uk, 2026)
www.gov.uk/ government/statistics/national-flu-and- covid-19-surveillance-reports-2025-to-2026- season WHO Writing Group, Ampofo WK, Baylor N, et al. Improving influenza vaccine virus selection: report of a WHO informal
consultation held at WHO headquarters, Geneva, Switzerland, 14-16 June 2010. Influenza Other Respir Viruses. 2013;7 Suppl 2(Suppl 2):52-53. doi:10.1111/irv.12081 World Health Organization. Global Influenza Programme - FluNet. (WHO, 2026) www.who. int/tools/flunet World Health Organization. Improved influenza vaccines: full value vaccine assessment. (WHO, 2026)
www.who.int/ publications/i/item/9789240117228 Zambon M, Hayden FG. Influenza A(H3N2) Subclade K Virus: Threat and Response. JAMA. 2026;335;(4):307-310. doi:10.1001/ jama.2025.25903
Professor Maria Zambon is a consultant virologist and head of the WHO National Influenza Centre (NIC) at the UK Health Security Agency (UKHSA) and has been the chair of the International Society for Respiratory Viruses (ISRV), an independent scientific society and registered charity, since 2020. Her main research interests include the diagnosis of viral infections in humans, especially RNA viruses, and the development of new vaccines and diagnostics for respiratory viruses, particularly influenza.
May 2026
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