Editor’s note: This story was updated to correct a county name on the CDC’s list.

(NEXSTAR) – Increased transmission of COVID-19 has landed more than a dozen California counties on the Center for Disease Control and Prevention’s watch list.

Thirteen counties in Northern and Central California have “high” community transmission, according to CDC data. It’s the first time since March that any county in the state has been in the “high” category, according to the Los Angeles Times.

The 13 California counties with high community levels of the virus are:

  • Del Norte
  • El Dorado
  • Marin
  • Mendocino
  • Monterey
  • Napa
  • Placer
  • Sacramento
  • San Benito
  • Santa Clara
  • Solano
  • Sonoma
  • Yolo

A county’s community level is based on the number of new COVID-19 cases, new hospital admissions, and the number of hospital beds filled with COVID patients. When a county has “high” community spread, the CDC recommends wearing a mask indoors, but it’s not a federal mandate.

In response to the rising numbers, one county has decided to reinstate its indoor mask mandate. As of Friday, face coverings are once again mandatory in Alameda County grocery stores, offices, and other indoor gathering places.

“Rising COVID cases in Alameda County are now leading to more people being hospitalized and today’s action reflects the seriousness of the moment,” Alameda County’s health officer, Dr. Nicholas Moss, said in a press release announcing the reinstated mask rules. “We cannot ignore the data, and we can’t predict when this wave may end. Putting our masks back on gives us the best opportunity to limit the impact of a prolonged wave on our communities.”

The CDC updates its map of community transmission levels every Thursday. If the spread of the virus worsens, several more California counties that are currently labeled as “moderate” could be bumped up, including San Diego and Los Angeles.