Subclade K Flu Variant Spreads Early in Japan, Raises Concerns for U.S. Flu Season
TOKYO — December 3, 2025 — A new influenza variant known as Subclade K has emerged early in Japan’s flu season and is spreading in other countries including Canada and the United Kingdom, officials said. The variant, a subtype of the type A flu virus, has been identified in a majority of recent flu cases in Japan and is beginning to appear in the United States as the 2025 cold and flu season approaches.
According to the Japan Institute of Health, 22 out of 23 patient samples collected since September tested positive for the Subclade K variant, representing a 96% detection rate. Despite its rapid spread, the variant is not believed to cause more severe symptoms or be significantly more contagious than other flu strains.
The Japanese health ministry reported 196,895 flu cases across approximately 3,800 medical facilities through November 23, averaging more than 51 patients per facility. This figure surpasses the threshold of 30 patients per facility required to issue a national flu warning and is substantially higher than the average of 2.36 patients per facility recorded during the same period last year.
The early onset of the flu season in Japan is partly attributed to the timing of the Subclade K variant’s arrival coinciding with the period when most citizens receive their flu vaccinations. This overlap meant many individuals contracted the virus before developing immunity from the vaccine.
Health experts have noted that the type A flu virus typically dominates the early part of the flu season, with type B viruses becoming more prevalent later. The early activity of Subclade K may influence flu patterns in other countries, including the United States, particularly among those who have not yet received their annual flu shots.
Officials continue to monitor the situation closely as the flu season progresses, emphasizing the importance of vaccination to reduce the risk of infection and complications.
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