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H3N2 Subclade K Fuels Early U.S. Flu Surge With Millions Sick

Officials report preserved protection against severe disease despite a partial vaccine mismatch.

Overview

  • CDC estimates at least 4.6 million illnesses, 49,000 hospitalizations and 1,900 deaths so far this season in the U.S., including pediatric fatalities.
  • Genetic testing shows roughly 89–90% of recent H3N2 samples belong to subclade K, which experts say carries mutations that help it evade some immunity.
  • Early U.K. data suggest vaccines remain effective against severe outcomes, with about 70–75% protection in children and roughly 30–40% in adults.
  • Health systems are reporting increasing strain heading into the holidays, and authorities urge flu shots for everyone 6 months and older.
  • Clinicians recommend prompt antivirals such as oseltamivir or baloxavir for high‑risk patients and basic precautions like staying home when sick and wearing a mask in crowded settings.