Eurosurv. Cross-sectional study on protective antibodies against influenza A virus subtypes and cross-protection against influenza A(H3N2) subclade K

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• Raquel Guiomar[SUP]1[/SUP] , Camila Henriques[SUP]1[/SUP] , Susana Pereira da Silva[SUP]2[/SUP] , Licínia Gomes[SUP]1[/SUP] , Daniela Dias[SUP]1[/SUP] , Nuno Verdasca[SUP]1[/SUP] , Portuguese Laboratory Network for the Diagnosis of Influenza and Respiratory Viruses[SUP]3[/SUP] , Baltazar Nunes[SUP]4[/SUP] , Ana Paula Rodrigues[SUP]2[/SUP]

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The 2025/26 season was marked by co-circulation of influenza A subtypes, with the first detection of A(H3N2) subclade K in September 2025. In August 2025 in Portugal, 14.8% (95% CI: 12.2–17.8) of 886 persons tested had cross-protective antibodies against this subclade. The overall seroprevalence against circulating A(H1N1)pdm09 strains was 28.1% (95% CI: 24.4–32.0). These data highlight the presence of previous cross-reactive antibodies and the possible advantage of vaccination in the extent of detectable antibodies against influenza viruses.

https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2026.31.6.2600070