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.

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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,

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