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.

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 Kingdom, which has seen a wave of severe infections earlier than expected. Health officials have warned that the U.K. is facing one of its wors

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