California announced it is reinstating its indoor mask mandate for everyone regardless of their vaccination status. The state health department says the new order will go into effect on Monday and last through Jan 15, with the possibility to be extended.

The Washington Examiner reports:

“As we look at the evidence that masks do make a difference, even a 10% increase in indoor masking can reduce case transmission significantly,” said State Health and Human Services Secretary Dr. Mark Ghaly.

The state has always required unvaccinated people to wear masks indoors, but rules for fully vaccinated residents vary by county (roughly 50% of Californians are not currently under a mask mandate). Ghaly told reporters the renewed mandate comes after cases in the state ballooned by 47% in the weeks following Thanksgiving.

In addition to mandatory masking, those attending large gatherings of 1,000 or more people who are not vaccinated will have to show proof of a negative COVID-19 test within one day of the event. Previously, people were permitted to show proof of a negative test within 72 hours. The state also recommends people traveling in and out of California be tested within three to five days.

The state anticipates an increase in case rates this month as people gather for holiday celebrations, particularly indoors where transmission occurs more easily. The state’s test positivity rate has climbed in recent weeks from about 1% of tests coming back positive on Nov. 24 up to more than 4% on Dec. 10, according to tracking from Johns Hopkins University

California is the latest state to reimpose the controversial rule since the discovery of the omicron variant.