New York City will impose a first-in-the-nation vaccine mandate on private companies, in what Mayor Bill de Blasio called a "preemptive strike" against the Omicron variant and a possible winter surge of COVID-19.

The mayor went on MSNBC's "Morning Joe" Monday morning to announce the mandate, which he said will take effect in three weeks, on Dec. 27.

“We in New York City have decided to use a preemptive strike to really do something bold to stop the further growth of COVID and the dangers it’s causing to all of us,” de Blasio said. “All private-sector employers in New York City will be covered by this vaccine mandate as of Dec. 27.”

New York City already requires vaccinations for hospital and nursing home workers, as well as city employees including teachers, police officers, and firefighters. Last week, the mayor's office also announced these mandates would be extended to private and religious schools.

This new mandate covering private businesses comes at the same time federal courts have temporarily blocked President Joe Biden's federal vaccine mandate on companies with more than 100 employees from taking effect.

At a press conference Monday morning, New York City Health Commissioner Dr. Dave Chokshi said that the city will issue the new vaccination rules on Dec. 15, but he did not give details on how they will be enforced.

“Vaccines work, and vaccine mandates work, particularly when joined with efforts to build vaccine confidence, provide incentives, and improve access, as we have in New York City,” Chokshi said. “We’ve seen this with our health care workers, school staff and public employees. Now it’s time for the private sector to step up and follow suit.”

New York City Councilman Mark D. Levine, the chairman of the council's health committee, said the mandate will include a weekly testing option, according to WLNY-TV.

The councilman added on social media that vaccination passport requirements at indoor public establishments like restaurants and theaters will be extended to include children ages 5 to 11 on Dec. 14 and will require a second vaccine shot as of Dec. 27.

Also, beginning on Dec. 14, children ages 5 to 11 will need to have at least one vaccine dose to participate in "high risk" extracurricular activities, including football and volleyball.

But all of these COVID-19 policies are subject to review once Mayor-elect Eric Adams takes over on Jan. 1. In a statement to WLNY-TV, the mayor-elect said he will "evaluate" the new mandate once he takes office.

“The Mayor-elect will evaluate this mandate and other COVID strategies when he is in office and make determinations based on science, efficacy and the advice of health professionals,” a spokesperson said.