Flu season in the U.S. is off to an early start—which means a virulent winter may be impending.

Influenza activity has been higher than normal for four weeks in a row, which many experts consider the official mark of flu season. As of the week ending Nov. 30, 3.5% of visits to health care providers nationwide were related to influenza-like illness, according to the U.S. Centers for Disease Control and Prevention (CDC). Already, the country has seen about 1.7 million cases of the flu, 16,000 flu-related hospitalizations and 910 flu-related deaths, the CDC says. Six children have died so far.

So far this year, flu activity has mainly been clustered in the south, with low prevalence in most northeast and mid-Atlantic states. Diagnoses are rising in the midwest and western regions.

The last time the flu arrived this early was the 2003-2004 season, which turned into a fairly severe one; at its peak, 10% of U.S. deaths were flu-related.

The strain of influenza causing most illnesses this year is also atypical for this part of the calendar. Influenza B, which typically circulates later in the season, has been responsible for about 70% of the positive influenza tests collected so far by the CDC.

It’s possible that the early flu season means either that fewer people got early-season vaccinations this year, or that the vaccine is less effective than normal. But the CDC says it’s too soon to say how well the shot is working, and emphasizes that there’s still plenty of time to get one, since flu season typically lasts several months. A flu shot remains the best way to prevent catching and spreading illness.

The 2018-2019 flu season was long but relatively mild, especially compared to the severe year that preceded it. It’s too soon to say how the 2019-2020 flu season will compare—but doctors are bracing for a bad one.

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Write to Jamie Ducharme at jamie.ducharme@time.com.