SONOMA COUNTY (CBS SF) – Sonoma County became the latest in the Bay Area to loosen indoor mask rules in certain settings where all in attendance have proven to be fully vaccinated against COVID-19.

Health Officer Dr. Sundari Mase announced Thursday that masks can be optional in offices, gyms and fitness centers, employee commuter vehicles, religious gatherings and college classes under some situations. Gatherings must have 100 people or less, the setting must not be open to the general public and that negative COVID-19 tests are not a substitute for showing proof of vaccination.

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“Given that our case rate is now on the decline, it makes sense to loosen the requirement for certain stable groups of fully vaccinated people,” Mase said in a statement.

Officials stressed the exemption is voluntary and that companies and event hosts reserve the right to enforce universal masking regardless of vaccination status.

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Mase went on to say, “In general, we must continue to wear our masks in most indoor settings in order to limit the spread of the virus during the coming winter months.”

Sonoma joins Marin and San Francisco counties in relaxing indoor mask rules in certain settings. Contra Costa County officials plan to loosen some indoor mask rules starting November 1.

Federal and state mask mandates covering health care settings, K-12 schools and public transit remain in effect.

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Sonoma, along with other Bay Area counties, recently announced metrics to lift indoor the mask mandate entirely, which include 80% of the county being fully vaccinated, along with low case rates and low hospitalization rates. As of Thursday, 78% of residents 12 and older are fully vaccinated.