Officials in Alameda, Contra Costa, Marin, San Francisco, San Mateo, Santa Clara and Sonoma counties and the city of Berkeley issued the advisory July 16 "out of an abundance of caution" but have yet to issue a new masking mandate for shared indoor spaces like grocery and retail stores.
The health officials also stressed that fully vaccinated residents remain well-protected both from contracting the virus, including the currently observed variants, and from becoming seriously ill or dying.
Employees are encouraged to continue following the masking guidelines set by the California Division of Occupational Safety and Health and their employers. They are also encouraged to wear a mask indoors when their employers has not verified the vaccination status of their co-workers.
The recommendation comes on the heels of Los Angeles County issuing a new mask mandate for indoor spaces last week amid an increase in COVID-19 cases, almost exclusively among the unvaccinated, and the proliferation of the more-transmissible delta variant.
"The delta variant is spreading quickly, and everyone should take action to protect themselves and others against this potentially deadly virus," Alameda County Health Officer Dr. Nicholas Moss said in a statement.
While the Bay Area is one of the most vaccinated regions in the state and country, local health officials had recognized the potential for renewed mask requirements in recent weeks as the delta variant became more of a threat, particularly for unvaccinated people.
"The Bay Area is very, very diverse and there's hot spots, within the Bay Area, of unvaccinated populations," Marin County Deputy Health Officer Dr. Lisa Santora said earlier this month. "So we would expect to see, especially in more rural areas (and) consistent with other rural areas across the nation, that you'll see increased rates of cases."
The indoor masking recommendation, the county health officials said in a joint statement, is intended to help protect unvaccinated residents, who are orders of magnitude more likely right now to contract the virus and be hospitalized with serious illness.
Last Friday, Centers for Disease Control and Prevention director Dr. Rochelle Walensky went as far as to say the virus "is becoming a pandemic of the unvaccinated."
In Contra Costa County, the seven-day average of new cases per 100,000 residents was 1.7 for fully vaccinated residents as of July 8 and 16.1 per 100,000 for unvaccinated residents.
"The highly infectious delta variant is now the predominant strain in Contra Costa County," Contra Costa County Health Officer Dr. Chris Farnitano said. "While vaccines remain our best tool against COVID-19, masking in indoor and crowded outdoor settings will help us curb the spread of this latest wave of infection."
The coalition of health officers is expected to revisit the recommendation in the coming weeks after monitoring local case and hospitalization data.
State officials have also been hesitant, for now, to reissue mask and social distancing requirements statewide, instead deferring to local health agencies to do what they feel suits their constituents.
As of July 16, California's rate of new cases per 100,000 residents was at 5.4.