Flu cases in the U.S. are seeing a marked uptick, according to the most recent Centers for Disease Control and Prevention (CDC) data. The new numbers, which cover the period up to December 20th and were released Tuesday, show that more than 25% of tests reported to the system were positive for influenza. This rate is nearly double what last year’s flu season saw around this time. 

Hospitalization rates due to flu are also high—three times higher than in the CDC’s numbers for the 2023-2024 flu season around this time, and substantially higher than last flu season’s hospitalization peak, which wasn’t reached until February 2025. 

The primary culprit? A variant of influenza A, dubbed subclade K, that first appeared in the summer, after this year’s flu shot had already been designed. Subclade K differs enough from the viruses used to inform the shot—and from flu viruses people already have immunity to—that it’s spread swiftly and widely as winter has gotten under way.

In the U.K., subclade K drove a very early flu season, with cases starting more than a month earlier than epidemiologists usually expect. It helped provoke the declaration of a flu epidemic in Japan in October. In the US, flu cases started to tick up around the usual time, but the rate of new infections has been noteworthy, say experts. 

Read more: What to Know About the New ‘Subclade K’ Flu Variant

As of the latest report, twenty-nine U.S. states or territories, as well as Washington, D.C., and New York City, now have high or very high rates of influenza-like illness, according to the CDC. This measure doesn’t use virus-specific tests, but it’s a way of keeping track of flu or flu-like disease on a broader scale.

Early data from the U.K. suggest that although this year’s flu shot isn’t a perfect match for subclade K, it still provides protection against hospitalization. The vaccine was 70-75% effective in preventing hospitalization in children and about 30-40% effective in adults, a lesser but still substantial advantage. In the US, as of early December, less than half of Americans had received this year’s shot. 

"It's definitely not too late to get a flu vaccine if you haven't done it already," a medical officer in the CDC's influenza division told NPR. Pharmacies offering the vaccine can be found via Vaccines.gov.
As the U.K. season kicked off, experts noted that early flu seasons often mean tougher flu seasons. The rate of positive tests is still high in the U.K., but the latest data suggest it may be trending downward. In the U.S., it’s still too soon to tell when the peak will arrive.