Flu cases across the United States are rising sharply, according to the latest data from the Centers for Disease Control and Prevention (CDC). Figures released Tuesday, covering the period up to December 20, show that over 25% of tests reported were positive for influenza—nearly double the rate seen at the same point last year.
Hospitalizations are also climbing, with current rates three times higher than during the 2023–2024 flu season at this time. The spike has surpassed last season’s hospitalization peak, which did not occur until February 2025. Health officials attribute the surge primarily to a variant of influenza A known as subclade K. This strain first appeared in the summer, after the current flu vaccine was designed, and differs enough from earlier viruses that existing immunity offers limited protection.
Subclade K has already influenced flu seasons abroad. In the U.K., it triggered an early season, beginning more than a month ahead of the usual schedule. Japan declared a flu epidemic in October after the variant spread quickly. In the U.S., flu activity started around the typical time, but the speed and scale of the increase have drawn attention from experts. According to the CDC, 29 states and territories, along with Washington, D.C., and New York City, currently report high or very high levels of influenza-like illness. This measure captures flu and flu-like symptoms broadly, without requiring virus-specific testing.
Despite the mismatch between the vaccine and subclade K, early evidence from the U.K. suggests that the flu shot still offers meaningful protection, particularly against severe illness. In children, the vaccine is estimated to prevent 70–75% of hospitalizations, while in adults, effectiveness ranges from 30–40%. In the U.S., fewer than half of Americans had received this year’s vaccine as of early December, leaving many vulnerable as the season progresses.
“It’s definitely not too late to get a flu vaccine if you haven’t done it already,” said a CDC medical officer in the agency’s influenza division. Americans can locate vaccination sites at Vaccines.gov.
Health experts caution that early flu seasons can be especially severe. In the U.K., rates of positive tests remain high but may be starting to decline. In the U.S., it remains unclear when the season will peak, leaving hospitals and public health systems on alert.
The CDC and local health authorities continue to monitor the spread of subclade K closely, urging vaccination and standard preventive measures such as frequent handwashing, mask-wearing in crowded spaces, and staying home when ill.
As winter progresses, officials stress that timely vaccination and vigilance will be crucial in limiting severe cases and protecting vulnerable populations, particularly young children, older adults, and those with underlying health conditions.