Eurosurv. Reduced neutralising antibody responses against emerging 2025/26 influenza A(H1N1)pdm09 subclade D.3.1 and A(H3N2) subclade K viruses among

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• Niina Ikonen[SUP]1,*[/SUP] , Anu Haveri[SUP]1,*[/SUP] , Erika Lindh[SUP]1[/SUP] , Oona Liedes[SUP]1[/SUP] , Saimi Vara[SUP]1[/SUP] , Sari H Pakkanen[SUP]2[/SUP] , Anu Kantele[SUP]2,3[/SUP] , Tea Nieminen[SUP]4[/SUP] , Veli-Jukka Anttila[SUP]3[/SUP] , Hanna Välimaa[SUP]2[/SUP] , Merit Melin[SUP]1[/SUP] , Carita Savolainen-Kopra[SUP]1[/SUP] , Hanna Nohynek[SUP]1[/SUP]

During autumn 2025, drifted influenza A(H3N2) subclade K and A(H1N1)pdm09 subclade D.3.1. and D.3.1.1 viruses were detected in Finland. We assessed antibody responses against 2024/25 vaccine and 2025/26 epidemic influenza A strains among 46 Finnish healthcare workers before and after influenza vaccination with the 2024/25 vaccine; this vaccine included identical A(H1N1)pdm09 but different A(H3N2) strains compared with the 2025/26 vaccine. Neutralising antibody responses were markedly reduced against the A(H3N2) subclade K virus and titres for certain A(H1N1)pdm09 strains were reduced.
​https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2026.31.6.2600094#html_fulltext