19/12/2025
A new “Super Flu” variant is spreading globally. Here’s what to know.
A newly mutated variant of influenza A, known as H3N2 subclade K, is rapidly spreading worldwide and has become dominant in multiple Northern Hemisphere countries, raising concern about a potentially severe 2025–2026 flu season in the United States.
First detected in Europe after this season’s vaccine strains were selected, subclade K has seven new mutations that may help it evade immunity from current flu shots. The strain has already driven early, harsh flu seasons in Japan and the United Kingdom and is circulating in Canada, where experts warn of a mismatch between the vaccine’s H3N2 component and this new variant. In the U.S., surveillance data and genomic sequencing show that H3N2 subclade K is already present in most states and makes up the vast majority of analyzed H3N2 samples, although overall flu activity, while rising, is not yet at peak levels. H3N2 viruses are generally associated with more severe illness, especially in older adults and young children, and disruptions in CDC surveillance during the recent government shutdown have made real-time tracking more difficult.
Despite the mutation and imperfect vaccine match, experts stress that vaccination remains crucial because the flu shot still offers meaningful protection against severe disease, hospitalization, and death, and is a good match for circulating H1N1 and influenza B strains. Symptoms of the new subclade K strain appear similar to typical influenza A: sudden onset of fever, chills, body aches, headache, profound fatigue, congestion, runny nose, and cough.
Because these symptoms overlap with COVID-19 and other respiratory infections, clinicians recommend testing, including use of available at-home “3‑in‑1” rapid tests for flu A, flu B, and COVID-19.
References (APA style)
Centers for Disease Control and Prevention. (2025). Types of influenza viruses. In *Influenza (flu)*. U.S. Department of Health and Human Services.
Centers for Disease Control and Prevention. (2025). *Weekly U.S. influenza surveillance report (FluView), 2025–2026 season*. U.S. Department of Health and Human Services.