According to the latest figures from the Centers for Disease Control and Prevention (CDC), influenza cases across the United States are rising sharply. The data, released Tuesday and covering the period through December 20th, show that more than 25% of tests reported to the surveillance system were positive for influenza—nearly double the rate seen at this time last year.
Hospitalization rates for flu are also elevated, running three times higher than the CDC’s numbers from the previous season, a level that wasn’t reached until February 2025.
The main cause is a variant of influenza A called subclade K, which first appeared in the summer after this year’s flu vaccine had already been developed. Subclade K is different enough from the strains used to create the vaccine—and from the flu viruses that people already have immunity against—that it has spread quickly and broadly as winter has set in.
In the United Kingdom, subclade K triggered an early surge, with cases beginning more than a month sooner than epidemiologists typically anticipate. It contributed to an unusually premature flu season. In the US, flu cases started climbing around the expected time, but the speed of new infections has been remarkable, according to specialists.
Based on the most recent report, twenty-nine US states or territories, along with Washington, D.C., and New York City, are now showing high activity levels, the CDC says. This measurement doesn’t depend on virus-specific tests but provides a broader way to monitor flu and flu-like illness.
Early findings from the U.K. suggest that even though this season’s flu shot isn’t a perfect match for subclade K, it still provides benefit. The vaccine was 70-75% effective at preventing hospitalization in children and about 30-40% effective in adults—a smaller but still significant gain. US vaccination figures as of early December were not yet disclosed.
“It’s definitely not too late to get a flu vaccine if you haven’t done it already,” said a medical officer in the CDC’s influenza division. Pharmacies that offer the shot can be located through official resources. As the U.K. season began, experts noted that early flu seasons often translate to more severe ones. The test positivity rate remains high in the U.K., but the wave may be stabilizing. In the US, it’s still unclear when the peak will occur.
