Flu season has come early in the U.K. Cases began to tick up in October , more than a month ahead of when epidemiologists generally expect the season to start. Driving the surge is a new variant, and while flu viruses are always evolving, this one has racked up an impressive number of mutations relatively quickly.

“This one has evolved a bit more rapidly, with more changes than we normally see,” says Jamie Lopez Bernal, a consultant epidemiologist at the U.K. Health Security Agency (UKHSA).

What is subclade K?

The variant is known as influenza A subtype H3N2 subclade K–sometimes called “subclade K” for short—and it has been detected in Japan as well, where public health authorities declared a flu epidemic last month .

The mutations mean the variant is a bit different from the viral

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