A novel flu strain, identified as subclade K, is raising concerns about a potentially severe flu season across the nation, according to health experts.

This strain has already prompted Japan to announce an influenza epidemic, while the United Kingdom is experiencing an unusually early start to its flu season—a trend now evident on the other side of the Atlantic.

In the United States, flu cases have surged to levels typically observed in December, reports Cameron Wolfe, a medicine professor and infectious disease expert at Duke University.

“We are witnessing the beginning of an increase in mid-November that resembles patterns seen overseas,” Wolfe stated.

“While the strain doesn’t appear more severe, it seems to have bypassed some of our previous immunity, leading to a higher number of cases. This might explain why we’re observing them earlier in the season than usual,” he added.

Subclade K is a variant of the H3N2 influenza, which has dominated past flu seasons and co-circulated with an H1N1 last year. 

H3N2 variants tend to cause more severe symptoms like a high fever, fatigue, and severe body aches in comparison to H1N1 variants, which tend to be more mild, according to the Doctors Diagnostic Center. 

Hospitalizations for the flu during the 2024-2025 flu season were the highest they’ve been since 2010, according to the Centers for Disease Control and Prevention (CDC). Last year’s flu season killed 280 children, the highest number for pediatric deaths since the CDC began tracking them. 

Wolfe said the trend may continue this year. “Does [an earlier flu season] translate to more hospitalizations? I think the answer is, sadly, inevitably yes. We’re going to have to grapple with that on the front.” he added.  

The populations most vulnerable to the flu include pregnant people, small children, the elderly and those with prior health conditions. 

The Vaccine Alliance warned that subclade K is worrying due to mutations that occurred since the summer, when researchers developed a flu vaccine around another sub-variant of H3N2, called subclade J.2.

“It also means that the virus could have changed sufficiently to escape the immunity that has been built up from previous infections and vaccinations,” said Dr. Antonia Ho, clinical senior lecturer and consultant in infectious diseases at the University of Glasgow in the U.K.

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. 

“Children are at increased risk of severe flu, particularly those under five, and it’s most important for those under two,” said Andrew Pavia, a professor and pediatric infectious disease expert at the University of Utah.

“The thing that really concerns me most is the decreasing rates of influenza vaccination, particularly among children.”

William Schaffner, a doctor who teaches infectious disease at Vanderbilt University, said one cause of this trend could be the growing number of people who don’t trust public health experts, with Health Secretary Robert F. Kennedy Jr., a longtime vaccine skeptic, now leading federal health agencies. 

“The kind of vaccine disparaging information that we have been receiving from Washington, particularly over the last year, has been confusing, discouraging and has contributed to a lack of trust in public health,” he said. 

“We don’t claim the vaccine to be perfect, it’s not, [but] it works better at keeping you out of the hospital, the intensive care unit and helping keep you out of the cemetery.”

Early reports from the UK show that this year’s vaccine still provides some protection against the new variant. 

“We expect that the current vaccine will keep [being] effective [and] will maintain effectiveness against the virus that [circulates] every year,” director of the Pan American Health Organization Jarbas Barbosa said to The Hill.