According to the latest data from the Centers for Disease Control and Prevention (CDC), there is a noticeable increase in the number of flu cases in the United States. new numberswhich cover the period up to Dec. 20 and were released Tuesday show that more than 25% of tests submitted to the system were positive for influenza. That figure is nearly double last year's flu season around this time.
Hospitalization rates for influenza are also high—three times higher than CDC figures for flu season 2023-2024 around this timeand significantly higher than the peak of hospitalizations last flu seasonwhich was achieved only in February 2025.
The main culprit? The influenza A variant, called subclade K, first appeared in the summer after the flu shot had already been developed earlier that year. Subclade K is so different from the viruses used for vaccination, as well as from the influenza viruses to which people are already immune, that when winter comes, it spreads quickly and widely.
In Great Britain the K subclade ruled very early flu seasonand cases began more than a month earlier than epidemiologists usually expect. This helped provoke announcement of a flu epidemic in Japan in October. In the U.S., flu cases began rising around normal times, but the rate of new infections was remarkable, experts say.
Read more: What you need to know about the new subclade K flu variant
Twenty-nine U.S. states and territories, as well as Washington, D.C. and New York, currently have high or very high incidence of influenza-like illnessesAccording to the CDC. The measure does not use tests specific to the virus, but is a way to track influenza or influenza-like illnesses on a broader scale.
Early data from the UK suggests that although this year's flu jab is not ideal for subclade K, it still provides protection against hospitalization. The vaccine was 70-75% effective in preventing hospitalization in children and 30-40% in adults, a smaller but still significant benefit. In the US as of early December less than half of Americans have been vaccinated this year..
“It's definitely not too late to get a flu shot if you haven't already,” CDC influenza medical officer. told NPR. Pharmacies offering the vaccine can be found through Vaccines.gov.
As the UK flu season began, experts noted that early flu seasons often mean more severe flu seasons. The UK's test positivity rate remains high, but recent data suggests it may be trending downward. In the US, it is too early to say when the peak will occur.
