Low vaccination rates may also be contributing to this year’s sick season. As of late January, U.S. vaccine uptake was well below the CDC’s target of 70% coverage, with only about 44% of U.S. adults and children vaccinated. That’s on par with recent seasons for adults, but lower than normal for kids.

The worst flu season in years is swamping California, prompting a renewed surge in hospitalizations as officials warn the disease could continue circulating at high levels for weeks to come.

The U.S. winter virus season is in full force and driving more doctor's office visits than any in the past decade.