Cross-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.