A new turn in the already unpredictable story of seasonal flu has put health services on alert: H3N2 subclade K, a mutated branch of influenza A H3N2, is gaining ground in several Northern Hemisphere countries and showing signs of increased virulence and immune escape compared to the strains targeted by this season’s vaccine.

Laboratories have found that subclade K carries multiple mutations in the hemagglutinin protein the key the virus uses to attach to human cells reducing the antigenic match with the vaccine formulation selected months ago by the WHO. That mismatch could translate into lower protection against infection, though the vaccines still provide strong protection against severe illness and hospitalization.

The first major clusters of subclade K appeared late in the Southern Hemisphere season and quickly took hold in countries such as the United Kingdom, Japan, and Canada, where hospitals have reported rising influenza admissions. In the United States, surveillance is showing an early uptick in influenza A cases (predominantly H3N2 in recent reports), but delays and gaps in monitoring systems are complicating a full reading of the national picture.

For the most vulnerable groups older adults, young children, pregnant people, and those who are immunocompromised the combination of higher transmissibility and reduced vaccine match could mean a more severe season than usual. Epidemiology experts warn against underestimating the situation: emergency rooms and hospital beds could come under pressure if transmission continues unchecked.

What can readers do? Health authorities are emphasizing the same classic, effective measures: vaccination because even if not perfect, it reduces hospitalizations handwashing, staying home when symptomatic, and considering mask use in crowded indoor settings for those at risk. Local surveillance and early communication between labs and hospitals will be crucial in the coming weeks to determine whether subclade K becomes dominant or is overtaken by other variants.