Overview
- Health agencies report a genetically drifted H3N2 subclade (known as subclade K) driving earlier, heavier outbreaks in the U.K., Canada and Japan, with WHO noting it now predominates in parts of the Northern Hemisphere.
- U.S. flu activity remains relatively low but is rising, especially in children, and H3N2 is the leading strain, though limited surveillance leaves the domestic footprint of subclade K uncertain.
- U.K. preliminary effectiveness estimates indicate vaccines reduce hospitalizations by roughly 70%–75% in children and about 30%–40% in adults this season.
- The current vaccine targets nearby H3N2 groups rather than subclade K, which could lower protection against infection even as it still helps prevent severe outcomes.
- Flu shot uptake is trailing last year by about 2 million doses at retail pharmacies (26.5 million Aug–Oct), and hospitals are preparing for added pressure from overlapping respiratory threats, including ongoing measles and COVID-19.