California is reimposing a broad indoor mask mandate this week as the Omicron variant continues spreading, holiday gatherings proliferate, and as COVID cases and hospitalizations tick upward across the state.

California's Secretary of Health and Human Services Dr. Mark Ghaly gave a press conference Monday to address the state's overall COVID response, and he made the surprise — though not that surprising — announcement that the state was reimposing indoor masking rules beginning Wednesday, December 15, through January 15.

"We are proactively putting this tool of universal indoor masking in public settings in place to ensure we get through a time of joy and hope, without a darker cloud of concern and despair," Ghaly said. "Californians have done this before. And we believe we can do it again."

While there has been early evidence that the highly infectious Omicron variant causes less severe illness than Delta and other variants, its ability to spread among fully vaccinated people remains clear. Booster doses offer extra protection against the variant, said Dr. Anthony Fauci over the weekend, but we know locally that multiple boosted Kaiser staffers got infected with Omicron at a wedding in Wisconsin in recent weeks.

Ghaly cited a 47% surge in new COVID cases statewide since Thanksgiving, and the mask rules are intended to tamp down another winter surge.  Mask mandates have been lifted in many counties across the state, but some of the biggest urban counties have mostly reinstated them since the state lifted its mandate on June 15. The state continued to require masks on public transit, in taxis, in all healthcare settings, and at airports.

Hospitalizations have ticked slightly upward in the Bay Area since Thanksgiving from 360 on December 1 to 415 as of Sunday — an uptick of 15%. Statewide, hospitalizations have gone up 9% in that same two-weeks span.

In San Francisco, everyone has been mostly masked indoors since mask rules were reimposed in August. However as of mid-October, SF lifted mask-wearing rules for gyms and offices where everyone was fully vaccinated — and mask rules in bars and restaurants have been loosely enforced, at best.

Most other Bay Area counties, as well as San Francisco, have said that indoor masking would continue until a county could show over 80% vaccination of its residents, and its hospitalization were stable or declining, and its case numbers had been in the CDC's yellow tier for at least three consecutive weeks. The only county to reach these metrics so far, albeit briefly, was Marin County, which has also seen an uptick in cases over the last two weeks.

Solano County never reimposed a mask mandate when other local counties did, however the cities of Vallejo and Benecia issued their own.

Approximately 50% of state residents have been living without mask rules in many public settings for the past six months.

The order taking effect Wednesday also impacts events of 1,000 people or more in counties that haven't imposed vaccine mandates for such events. Attendance at those events will require either proof of vaccination, or proof of a recent negative COVID test — 24 hours for an antigen test, or 48 hours for a PCR test.

Ghaly also encouraged everyone traveling for the holidays to get a COVID test three to five days after they return to California — and for all travelers to California to do the same.

Ghaly did not provide any enforcement details, but he encouraged counties and businesses to enforce the rules themselves in an effort to "save lives," he said.

Update: As of December 15, the state announced that four Bay Area counties — San Francisco, Alameda, Contra Costa, and Marin — could continue with their own masking rules due to high vaccination rates and low hospitalizations, and no-mask gym-going could continue in these counties.

Top image: California Gov. Gavin Newsom (R) looks on as California Health Secretary Dr. Mark Ghaly speaks during a news conference after touring the vaccination clinic at City College of San Francisco on April 06, 2021 in San Francisco, California. (Photo by Justin Sullivan/Getty Images)