As COVID-19 cases and hospitalization rates rise again, Bay Area counties are tightening up health rules to require everyone — vaccinated or not — to wear masks inside public settings at all times.

Contra Costa, Alameda, Marin, Sonoma and San Francisco counties and the city of Berkeley announced this week that beginning Thursday, people will have to wear masks inside gyms, churches, offices and other places that had been exempted from mask rules as long as everyone was vaccinated..

The counties had eased their rules just weeks ago by allowing fully vaccinated groups of 100 or fewer people to remove their masks in “controlled spaces” such as workplaces and gyms.

But with the omicron variant spreading, public health officials in those areas decided to reverse course. California’s seven-day average for daily COVID cases per 100,000 people rose from 10 a month ago to 31.2 as of Tuesday, according to the state’s data update.

Studies so far have shown the omicron variant is even more contagious than the delta variant that preceded it, though possibly not as harmful. And health officials worry that the rising case numbers will put major stress on local hospitals, even if relatively few of those infected with coronavirus require stays.

“We have learned that Omicron can spread even among the vaccinated and those who had prior COVID infection,” Alameda County Health Officer Dr. Nicholas Moss said in a press release. “Masks work and are a critical layer of safety for everyone this winter.

“Because even a mild infection in a vaccinated person may be passed on to someone who could become hospitalized, we must take every precaution this winter,” Moss continued.

 Santa Clara County  announced Tuesday that workers in health care settings, nursing homes, homeless shelters and correctional facilities in the county must get boosters by Jan. 24. The day before, Contra Costa County announced it will require first responders and other high-risk workers to get their booster shots by Jan. 10.  San Francisco will require people to have their vaccine boosters at indoor events of at least 1,000 people, starting Feb. 1.

Santa Clara, San Mateo and Napa counties never rolled back their mask mandates, which were enacted primarily in response to the rise of the delta variant over the summer. Now, all Bay Area counties require masks in indoor public settings.