Washington Mayor Muriel Bowser, D, on Monday reimposed the city’s indoor mask mandate and announced a vaccination mandate for D.C. government workers – measures intended to slow the spread amid concerns over a surge in coronavirus caseloads and the rise of the highly transmissible omicron variant.

The city will again legally require masks in most indoor public places, such as gyms, churches and grocery stores, beginning at 6 a.m. Tuesday.

Bowser’s announcement comes just over a month after she said the city would greatly relax the indoor mask mandate that had been in place since July, asserting at the time that vaccines were effective and preventing many people from requiring hospitalization. About two weeks later, as the new omicron variant fueled anxiety about the virus, the city’s health department issued a mask “advisory” that strongly recommend – but did not legally require – unvaccinated and vaccinated people wear masks in all indoor settings.

On Monday, Bowser said the mask mandate would last until Jan. 31.

Ten of 13 members of the D.C. Council had called on Bowser to reimpose the city’s mask mandate soon after it was lifted in November, arguing that the decision was poorly-timed ahead of winter holidays and when many children were still not fully vaccinated.

Officials remained firm on their position at the time and said the city would not change course. But last week, when Bowser was asked about single-day coronavirus totals that had eclipsed levels the city had reported at any point during the pandemic, she said bringing back the mask mandate was “on the table.”

New coronavirus cases in the District have risen sharply in December. The seven-day average of new cases per 100,000 people was 51.01 on Friday compared to 10.9 at the beginning of the month; the seven-day average of new hospitalizations has grown from 87 on Dec. 1 to 169 on Friday – the highest rate of hospitalizations since May.

Bowser also said the District will soon require all the city’s 37,000 D.C. government workers to be fully vaccinated, including booster shots. In August, Bowser announced that all city employees and contractors would be required to get vaccinated, with an option for weekly testing in hopes that it would boost vaccination rates. But that test-out option is no more.

Officials said they still needed to partner with labor unions on the timing and specifics of the new requirement.

Starting Wednesday, rapid antigen coronavirus tests will be available to D.C. residents at eight public libraries, one in each ward. Residents will need to show valid D.C. identification or a piece of mail showing their D.C. address to receive the tests.

School officials have said that covid-19 infection rates increased in schools after the Thanksgiving holiday break. The city has moved four schools to virtual instruction through Dec. 22: Whittier Elementary School, McKinley Tech High School, Turner Elementary School and Bard High School Early College.

Other schools across the city have been prepping for another post-winter break covid-19 surge. KIPP DC, the city’s largest charter network, changed its plans last week: now students will be returning to virtual instruction and learning asynchronously on Jan. 3.

All of the staff and students at KIPP DC must report to the school and get tested for the coronavirus during the day through a rapid antigen test. Students will return in staggered arrival times the day Jan. 4, and only staff and students with a verified negative test will be admitted onto campus.