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Flu Season Accelerates as H3N2 Subclade K Becomes Dominant in the U.S.

Officials urge vaccination to reduce severe disease despite a partial mismatch.

Overview

  • The CDC estimates 4.6 million illnesses, 49,000 hospitalizations and 1,900 deaths so far this season, including pediatric deaths.
  • Hospital admissions rose about 14% week over week, with very high or high flu activity reported in states such as New York, Rhode Island, Colorado and Louisiana.
  • Nearly 90% of genetically analyzed H3N2 viruses belong to subclade K, which has drifted from the vaccine strain.
  • Experts report reduced protection against infection from this drifted strain, yet vaccines continue to lower the risk of severe outcomes, with stronger signals in children.
  • Health agencies recommend vaccination for everyone 6 months and older and early use of antivirals like oseltamivir or baloxavir for those at higher risk or with confirmed flu.