Contagious flu strain spreading. Here’s what you need to know about the mutation

by Ismael M. Belkoura, Fort Worth Report
December 21, 2025

A new mutant strain of the flu is spreading nationwide, which health officials warn may reduce the vaccine’s efficacy. 

The variant is a more contagious mutation of the influenza A(H3N2) virus, the most predominant version of the flu since September, according to the World Health Organization. 

Officials from the Centers for Disease Control and Prevention note this season’s flu vaccine “may be reduced” but still being effective in reducing the chances of hospitalization and death.

Since Sept. 28, 89% of flu viruses sampled by the CDC have been of the mutated subclade K strain.

Although North Texas hasn’t been hit hard yet, the mutation will certainly have an effect on the Lone Star state, said Dr. David Winter, a physician with Baylor Scott & White Health. 

“It’s worse in New York state, New Jersey, Louisiana — which is close by — in Colorado,” Winter said. “That’s where the most cases are now. But it’ll spread throughout the entire country, and you can be sure we’re going to have a lot of cases here in time to come.”

The CDC estimates there have been 2,900,000 cases, 30,000 hospitalizations and 1,200 deaths from flu this season as of Dec. 6. 

Where can you get the flu vaccine? Check out our previous coverage this year for information.

The flu season is a period during the year where the virus sees the highest activity. In the United States, that’s typically from October to May, with the highest number of cases reported December to February.

Based on the flu seasons of other countries, including Great Britain and Australia, Winter said “it should be a pretty bad year this year for influenza” in the United States.

“Last year’s flu season was bad. This one, from experience overseas, may be even worse,” the doctor cautioned. “So this is not something you want to ignore.”

The CDC recommends everyone over 6 months receive the annual flu vaccine.

For those who choose not to receive the vaccine and end up getting the flu, Winter emphasized the need to receive medication to prevent a severe case.

“The best way to prevent getting the flu is to get a flu vaccination,” he said. “But if you don’t get one, the second best thing is to get an early diagnosis so we can get some pills to keep you out of the hospital.” 

Ismael M. Belkoura is the health reporter for the Fort Worth Report. Contact him at ismael.belkoura@fortworthreport.org. 

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