A new type of seasonal flu virus is raising concerns among health officials as winter approaches.
Scientists are closely watching H3N2 subclade K, a newly emerged variant that is genetically distinct from the strain selected for this year’s flu vaccine.
There's concern that the new variant could at least partially escape immunity built from past infections and vaccinations, said Dr. Antonia Ho, a clinical senior lecturer and infectious disease consultant at the University of Glasgow.
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Subclade K was first detected late in the Southern Hemisphere’s flu season and is now dominating samples in the United Kingdom and Japan. UK data shows the flu season began about five weeks early, with H3N2 making up most cases, and nearly all of those belong to subclade K. The variant has also been identified in North America, though it’s unclear how common it is.
There is a major concern because health officials say H3N2 viruses also tend to cause more severe illness, especially in older adults.
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Despite the shift, vaccination is still the best way to protect yourself. Early UK estimates show the 2025–26 flu vaccine is 70% to 75% effective in children and 30% to 40% effective in adults, within the typical range for seasonal vaccines.
“It remains our best defense against serious illness,” said Dr. Jamie Lopez Bernal of the UK Health Security Agency.
