A new flu variant, subclade K, threatens to worsen the upcoming flu season, raising concerns among health experts.
Story Highlights
- Subclade K, a new flu strain, could lead to a severe flu season in the U.S.
- This variant has already caused an influenza epidemic in Japan.
- Early upticks in flu cases are being reported across the United States.
- Lower vaccination rates in the U.S. could exacerbate the situation.
Subclade K: A New Threat
Subclade K, a variant of the H3N2 influenza, has emerged as a significant threat for the 2025 flu season. Public health experts are voicing concerns as the strain has already caused Japan to declare an influenza epidemic. In the United Kingdom, the flu season started earlier than average, reflecting a worrying trend likely to hit the United States.
Cameron Wolfe, an infectious disease specialist at Duke University, notes that U.S. flu cases have reached levels typically seen in December, suggesting an increased risk of widespread infection.
“We’re here mid-November (2025) seeing the beginning of an uptick that’s actually mirrored by what we’ve seen overseas,” Wolfe said. “It’s not necessarily showing any signs of being more severe, I think it does look as if it has sort of escaped some of our prior immunity, and therefore we’d expect more cases, and maybe that’s why we’re seeing them more early in the season than what we would usually see.
Why ‘subclade K’ could make for a nasty flu season https://t.co/DacCDaRQo7
— The Hill (@thehill) November 23, 2025
Implications for Public Health
The implications of subclade K are alarming, particularly for vulnerable populations such as the elderly, pregnant individuals, small children, and those with pre-existing health conditions.
The Vaccine Alliance has expressed concern about mutations in the strain since the summer, which may allow it to evade immunity from previous infections or vaccinations.
Andrew Pavia, a pediatric infectious disease expert at the University of Utah, emphasizes the increased risk for young children under five, especially those under two, who are more susceptible to severe flu symptoms.
On top of a new strain, Americans aren’t getting vaccinations as much as they were in previous years, though the latest CDC data on flu vaccinations is from April. The trend of decreased vaccination rates is troubling, particularly among children, and could lead to higher hospitalization rates, echoing the severe 2024-2025 flu season, which saw the highest hospitalizations since 2010.
Public Trust and Vaccination
William Schaffner, an infectious disease specialist at Vanderbilt University, attributes part of the declining vaccination rates to a lack of trust in public health authorities, exacerbated by conflicting information from federal health agencies under the current leadership.
Despite the challenges, early reports from the U.K. suggest that the available vaccines may still offer some protection against subclade K. Jarbas Barbosa, director of the Pan American Health Organization, remains hopeful that the current vaccines will continue to be effective in reducing severe outcomes associated with the flu.
“We expect that the current vaccine will keep [being] effective [and] will maintain effectiveness against the virus that [circulates] every year,” Jarbas Barbosa, director of the Pan American Health Organization, said.
