After easing its COVID-19 health rules last month to allow vaccinated groups of people to ditch their masks in certain indoor settings, Alameda County reversed course this week.

On Wednesday, the county issued an order saying people must wear masks indoors at workplaces and other public settings.

Though it didn’t issue an announcement, the county posted on its website a health order dated Dec. 8 and signed by Health Officer Nicholas Moss, that requires people in Alameda County to don their masks indoors at offices, stores, restaurants and bars, theaters, conference and event centers, and state or local government offices that are open to the public.

The order stipulates that people do not have to mask up while working alone in a closed office or room, while eating or drinking, while receiving religious sacraments where a face covering would get in the way, and while swimming or showering in a gym. The order also carves out exemptions for those who play sports indoors “where masks create imminent risk to health (e.g., wrestling, judo),” and for those getting a medical or cosmetic service that requires the removal of a mask.

People performing onstage in musical or other events are also exempted from the order, as are those with medical conditions, disabilities or mental health issues that make it risky to wear a mask.

Representatives of the county’s health department did not return requests for more information about the order and whether it had been relayed to businesses.

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But it appears to be a reversal of its Nov. 1 order that lifted mask mandates for “controlled spaces not open to the general public” such as gyms and fitness centers, offices and religious services.

Under that order, businesses and other organizations had to confirm that workers, patrons or attendees were fully vaccinated — and that no more than 100 people were in a facility at once — before forgoing the mask requirement.

The new health order requires all people — vaccinated or not — to comply.

“Despite high vaccination rates, the County is experiencing substantial levels of community transmission due to the Delta variant,” the order reads. “While the risk for COVID-19 infection is highest among unvaccinated residents, the incidence of infection among fully vaccinated persons is increasing.”

According to information on its COVID-19 website, Alameda County confirmed 1,409 cases over the previous 14 days leading up to Dec. 7 — the latest available data. By that date, almost 75% of eligible people in the county had been vaccinated, and there was still what health officials considered a “substantial” level of virus transmission in the community.