A new variant of influenza H3N2 is spreading in other countries. But that could mean the U.S. is in for a rough flu season.

The mutated strain, known as subclade K, is causing a surge in cases in the United Kingdom, Canada, and Japan.

While H1N1 predominated flu season this year in the Southern Hemisphere, with the H3N2 subclade K taking off only at the end, early data from the UK and Japan show H3N2 subclade K was represented in 90% of flu samples, the University of Minnesota wrote in a report earlier this month.

With a new strain spreading and less Americans getting vaccinated, the sick season might be brutal.

Here's what to know.

Where has the H3N2 subclade K strain been detected?

According to the European Centre for Disease Prevention and Control , the strain has been detec

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