More than 25% of influenza tests reported to the Centers for Disease Control and Prevention (CDC) came back positive as of December 20th, signaling a significant surge in flu cases across the United States. That’s nearly double the rate seen around this time last year, and experts are bracing for a potentially challenging winter.
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A new influenza A variant, dubbed subclade K, is fueling a rapid increase in infections and hospitalizations.
- Flu cases are rising sharply, with over 25% of tests positive as of December 20th.
- Hospitalization rates are three times higher than this time last year.
- A new variant, subclade K, is largely responsible for the uptick, as it differs from viruses used in this year’s flu shot.
- While the current vaccine isn’t a perfect match, it still offers substantial protection against severe illness.
Hospitalization rates due to the flu are also concerningly high—three times higher than reported by the CDC for the 2023-2024 flu season around this time, and substantially higher than last flu season’s peak, which wasn’t reached until February 2025.
The Rise of Subclade K
The primary driver behind this increase is a variant of influenza A, known as subclade K. This variant first emerged during the summer, after the formulation of this year’s flu vaccine had already been finalized. Subclade K exhibits enough differences from the viruses used to create the vaccine—and from viruses to which people already have immunity—that it has spread quickly as colder weather arrived.
The impact of subclade K isn’t limited to the U.S. In the United Kingdom, it triggered a very early flu season, with cases beginning more than a month ahead of schedule. The variant also contributed to the declaration of a flu epidemic in Japan in October. While cases in the U.S. began to rise around the typical time, the speed of new infections has been particularly noteworthy, according to health experts.
Is it too late to get a flu shot? Experts say no. Even with the emergence of subclade K, the current vaccine still provides significant protection against hospitalization and severe illness.
As of the latest CDC report, twenty-nine U.S. states or territories, along with Washington, D.C., and New York City, are experiencing high or very high rates of influenza-like illness. This metric, while not based on virus-specific tests, provides a broad overview of flu activity.
Vaccine Effectiveness and Uptake
Early data from the U.K. indicate that, despite not being a perfect match for subclade K, this year’s flu shot still offers important protection against hospitalization. The vaccine demonstrated 70-75% effectiveness in preventing hospitalization among children and approximately 30-40% effectiveness in adults—a substantial benefit. However, as of early December, less than half of Americans had received this year’s flu 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 located via Vaccines.gov.
Experts have observed that early flu seasons often foreshadow more severe ones. While the rate of positive tests remains high in the U.K., recent data suggest it may be beginning to decline. In the U.S., it’s still too early to predict when the peak of the season will arrive.
