This recommendation follows similar guidance from the U.S. Centers for Disease Control and Prevention on Tuesday, recommending that even the vaccinated wear masks in areas where the virus is spreading more rapidly.
RELATED: CDC reverses course on indoor masks in some parts of US
According to the California Department of Health, more than 90% of California's population resides in areas with "substantial or high transmission" of the Delta variant.
"The Delta variant has caused a sharp increase in hospitalizations and case rates across the state," Dr. Tomás J. Aragón, California's public health officer, said in a statement.
In the Bay Area, four counties are seeing "high" rates of transmission. This includes Alameda, Contra Costa, San Francisco and Solano counties.
The five other Bay Area counties have what is considered "substantial" transmission.
RELATED: Doctors say the Bay Area 'could avoid' another lockdown if mask mandate is reinstated
In Los Angeles County, health officials reinstated a mask mandate earlier this month, just one month after the state lifted its mask order.
Earlier this week, Gov. Gavin Newsom announced that state workers will be required to be vaccinated or undergo mandatory COVID-19 testing and wear a mask.
The state's vaccination verification program includes health care workers as well. If health care workers are not vaccinated, they will be tested for COVID-19 twice per week and asked to wear N95 masks.
According to the most recent data, California reported 6,849 new cases of COVID-19 on Tuesday and another 38 deaths related to the virus.
The state reports that about 53% of residents are fully vaccinated against COVID-19, with 61% receiving at least one dose.
VACCINE TRACKER: How California is doing, when you can get a coronavirus vaccine
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