Flu season isn't in full swing yet in the United States, but a mutated strain of the virus that has caused severe outbreaks abroad is sparking concern about the winter ahead.

A new version of influenza A H3N2 emerged earlier this year after picking up a number of mutations, which may improve its ability to escape immunity from vaccines, experts say.

The mutated H3N2 strain, called “subclade K,” has spread rapidly and gained dominance in several countries in the Northern hemisphere.

It caused a surge in infections in Japan , which declared an influenza epidemic after experiencing an unusually early and harsh flu season, Dr. Robert Hopkins Jr., medical director of the National Foundation for Infectious Diseases, tells TODAY.com.

The subclade K strain is also sweeping the United Kingdo

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