This year’s flu season is shaping up to be one of the most punishing in recent memory, with hospitalizations climbing sharply and the CDC estimating 4.6 million flu cases nationwide so far—even as peak season is still weeks away.

Health officials say the surge is being driven by an unusually aggressive strain of influenza A, known as H3N2 subclade K, a variant that has quickly become dominant and is leaving doctors alarmed by both the speed and severity of illness. As of mid-November, the strain accounted for more than half of all flu cases in the United States, and there is little indication it is slowing down.