biostudies-literatureUnknown31(6)Guiomar RThe 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.Euro surveillance : bulletin Europeen sur les maladies transmissibles = European communicable disease bulletinhttps://www.ebi.ac.uk/biostudies/studies/S-EPMC12905529biostudies-literatureCross-sectional study on protective antibodies against influenza A virus subtypes and cross-protection against influenza A(H3N2) subclade K, Portugal, August 2025.PMC12905529Rodrigues RSancho LLoureiro AFonseca PDias DHenriques CAgostinho SRodrigues FAlves JAbreu GKronberg EGama PMoita CAliyeva EPereira PLopes PPereira da Silva SPeres JPortuguese Laboratory Network for the Diagnosis of Influenza and Respiratory VirusesSoares MJVaz CMajar MFrota AReis TMartins NSAlmeida SFalcao MPCunha MSilva ACMota PMota Freitas JAMota JMira ALuis JDias MTNogueira BRodrigues CSerrano ASilva HNunes DGomes SRamos MJGuiomar RSilva LDias APMoreira ILouro AMoreira ENunes BRodrigues Moreira JACorreia SCalheiros AFSeabra SLopes AHCaldeira FFradilha MToscano Cde Varzim PGomes LFreitas LRego TGCardoso MJMarques ACouto ARMarques BVerdasca NAmantegui PPedrosa AVieira SMCoimbra LPires ACastro APFerreira CLopes CFerreira AMontanha MJPaulo SGoncalves AGuimaraes JTRodrigues APRamos AMesquita ABarbeiro PMarques MHCosta TMagalhaes CfalseCross-sectional study on protective antibodies against influenza A virus subtypes and cross-protection against influenza A(H3N2) subclade K, Portugal, August 2025.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.2026-01-01T00:00:00Z2026 Feb2026-07-15T16:37:24.482Z2026-07-07T03:08:23.628ZS-EPMC129055294168432210.2807/1560-7917.ES.2026.31.6.2600070