Compared to previous years, influenza is increasing unusually early in the EU/EEA, with A(H3N2) driving the increases in recent weeks. This situation reflects developments recently reported by other northern hemisphere countries.
The newly-emerged A(H3N2) subclade K (former J.2.4.1) has been now detected on all continents and accounts for a third of all A(H3N2) sequences deposited in the Global Initiative on Sharing All Influenza Data (GISAID) between May and November 2025 globally, and almost half in the EU/EEA. Phylogenetic analysis shows a significant divergence of subclade K from the northern hemisphere A(H3N2) vaccine strain. In-vitro antigenic and serological analyses also suggest a mismatch between the vaccine and this new subclade. Real-world vaccine effectiveness data are currently limited.
A(H3N2) has not been the dominant virus in recent seasons which may lead to lowered immunity in populations without recent exposure, although serological data are not yet available to assess this further. Countries in east Asia who now report declining epidemics of A(H3N2) have not experienced unusually high disease severity, and phylogenetic analysis suggest that the A(H3N2) subclade K strains circulating in these countries are no different from those present in the EU/EEA.
Even if a less well-matched A(H3N2) virus dominates this winter, the vaccine is still expected to provide protection against severe disease, so it remains a vital public health tool.