Flu season is officially underway in the U.S . — a time marked by cold weather, tissues, aches and pains.

While coming down with the flu is never ideal, this year a newly emerged version of Influenza A H3N2 — known as subclade K — could make this winter’s flu wave tougher than usual.

What is H3N2 — and what is subclade K?

Influenza A H3N2 is one of the main seasonal flu subtypes globally and is often associated with more severe illness compared to other flu strains.

Subclade K is not a new type of flu, but rather a mutation of Influenza A H3N2 that has established a dominance this flu season.

According to the Centers of Disease Control , subclade K was identified in the U.S. back in August and has continued to gain momentum since.

In September, 86.6% of the nation’s 91 cases were

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