On December 4, 2025, the World Health Organization (PAHO/WHO) reported through its epidemiological alert that, in recent months, the situation of the circulation of the influenza A(H3N2) virus related to the K subclade has increased in several regions of the world, especially in Europe and in several countries in East Asia, and reiterated its recommendations in the region of the Americas.

The A(H3N2) subclade K viruses have been genetically derived from the related J.2.4 viruses, and in a process of natural evolution have accumulated several amino acid changes in their hemagglutinin, compared to the previously circulating J.2.4 viruses.

In Europe, influenza activity began earlier than usual and subclade K represented almost half of the sequences reported between May and November 2025. To date, no significant change in clinical severity, in terms of hospitalization, intensive care admissions, or deaths, has been documented. However, seasons dominated by the A(H3N2) subtype are usually associated with greater severity, especially among older people.

In the region of the Americas, specifically in the subregion of North America (5–7), a sustained increase was observed, mainly due to influenza type A. Indeed, in the United States of America and Canada, greater circulation of subtype A(H3N2) was recorded, with a progressive increase in detections of subclade K of influenza A(H3N2).

East Asian countries, now seeing declining circulation of influenza A(H3N2), have not reported increased disease severity to date. Phylogenetic analyzes of subclade K strains circulating in these countries suggest that they are not different from those present in European countries.