New York City sets a sweeping vaccine mandate for all private employers.

Mayor Bill de Blasio announced a sweeping coronavirus vaccine mandate for all private employers in New York City on Monday morning to combat the spread of the Omicron variant.

Mr. de Blasio said the aggressive measure, which takes effect Dec. 27 and which he described as the first of its kind in the nation, was needed as a “pre-emptive strike” to stall another wave of coronavirus cases and help reduce transmission during the winter months and holiday gatherings.

“Omicron is here, and it looks like it’s very transmissible,” he said in an interview on MSNBC. “The timing is horrible with the winter months.”

New York City has already put vaccine mandates in place for city workers and for employees and customers at indoor dining, entertainment and gyms. Nearly 90 percent of adult New York City residents now have at least one dose of the vaccine.

But Mr. de Blasio said the city must go further to combat another wave of the virus in New York City, once the center of the pandemic. Some private employers have required employees to get vaccinated, but many others have not.

Mr. de Blasio said the new measure would apply to about 184,000 businesses. Employees who work in-person at private companies must have one dose of the vaccine by Dec. 27; remote workers will not be required to get the vaccine. There is no testing option as an alternative.

The city plans to offer exemptions for valid medical or religious reasons, Mr. de Blasio said. City officials will release detailed guidelines about issues like enforcement by Dec. 15 after consulting with business leaders.

The mayor also announced that the rules for dining and entertainment would apply to children ages 5 to 11, who must have one dose to enter restaurants and theaters starting on Dec. 14, and that the requirement for adults would increase from one dose of a vaccine to two starting on Dec. 27, except for those who initially received the one-shot Johnson & Johnson vaccine.

Mr. de Blasio and Gov. Kathy Hochul held a news conference last Thursday to announce New York State’s first five cases of the Omicron variant, and several more have been announced in New York City since then. The number of coronavirus cases in the city has increased rapidly in recent weeks; daily case counts have increased more than 75 percent since Nov. 1.

Mr. de Blasio, a Democrat with less than a month left in office, said he was confident the new mandate would survive any legal challenges and he noted that past city mandates had been upheld.

“They have won in court — state court, federal court — every single time,” the mayor said on MSNBC. “And it’s because they’re universal and consistent.”

Eric Adams, the mayor-elect who takes office on Jan. 1, is on vacation in Ghana this week. His spokesman, Evan Thies, said in a statement that Mr. Adams would evaluate the measure once he is mayor.

“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,” he said.

The Biden administration tried to set a federal mandate that all large employers must require workers to get vaccinated or submit to weekly testing starting in January, but that measure is stalled in court.

The Occupational Safety and Health Administration, or OSHA, issued an “emergency” rule earlier this month requiring vaccinations for employees of companies with at least 100 workers, although it exempts those who work at home or exclusively outdoors.

Georgia M. Pestana, New York City’s corporation counsel, said at a news conference on Monday that the city’s health commissioner clearly has the legal authority to issue a mandate to protect New Yorkers during a health crisis. She argued that the legal questions over the Biden administration’s mandate were different and centered on whether OSHA had the proper authority.

Kathryn Wylde, the president of a prominent business group, the Partnership for New York City, said she was surprised by the announcement by Mr. de Blasio.

“We were blindsided,” she said. “There’s no forewarning, no discussion, no idea about whether it’s legal or who he expects to enforce it.”

Roughly half of Manhattan office employers have enacted vaccine mandates, she said, though some policies include testing options and medical and religious exemptions.

Many questions remain about the Omicron variant. Some early signs exist that it may cause only mild illness, though that observation was based mainly on cases in South Africa among young people, who are generally less likely to become severely ill from Covid. Scientists are also waiting to see whether cases lead to substantial hospitalizations and deaths; both are lagging indicators.

And at the moment, scientists say there is no reason to believe Omicron is impervious to existing vaccines, although they may turn out to be less protective to some unknown degree.