Threat Assessment Brief: Assessing the risk of influenza for the EU/EEA in the context of increasing circulation of A(H3N2) subclade K
Circulating respiratory viruses, including influenza viruses, SARS-CoV-2 and RSV, all contribute to pressure on healthcare systems during winter in the EU/EEA. In a typical season, influenza causes substantial morbidity in the European population, with up to 50 million symptomatic cases and 15 000 to 70 000 deaths annually. All age groups are affected, although children have higher rates of illness and are usually the first to become sick and transmit the disease in their households, which can drive transmission in the community. It is estimated that up to 20% of the population contract influenza annually. This results in absence from school and work and a significant impact on healthcare systems. A higher impact is seen in closed settings such as long-term care facilities (LTCFs), where outbreaks of seasonal influenza can have high morbidity and mortality.
ECDC decided to assess the risk of influenza for the EU/EEA in the context of early circulation of seasonal influenza in the region and the recently emerged influenza A(H3N2) subclade K that is circulating globally. This is to raise awareness of potential implications and provide recommendations to public health authorities. However, considerable uncertainty remains around the likely public health impact of this subclade on the influenza season.
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Epidemiological situation
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.
