All private-sector employees in New York City will be required to get at least one dose of COVID-19 vaccine by December 27th, part of what Mayor Bill de Blasio called a "preemptive strike" against a potential winter surge.

The mayor made the announcement on Monday morning during an appearance on MSNBC. He said the emergence of the omicron variant coupled with colder weather and holiday gatherings prompted him to issue what would amount to the broadest vaccine mandate in the country.

De Blasio said Health Commissioner Dr. Dave Chokshi would release full details of the new order. Currently, all municipal workers, health care providers and education staff are required to get the shots, while private employers are left to their own discretion.

"It looks like it's very transmissible," de Blasio said of the omicron variant. "It’s just going to make a tough situation even tougher."

Listen to Elizabeth Kim's report on the city's vaccine mandate announcement for private sector employees on NPR:

De Blasio, whose mayor term ends four days after this policy takes effect, has previously said he would be willing to impose more mandates to fight the pandemic. Last week, he issued new vaccine mandates for private and religious school employees and childcare workers.

The mayor also said he plans to change his Key to NYC vaccine requirements for restaurants, gyms and various cultural and entertainment venues. Starting on December 27th, at least two doses of the vaccines rather than one will be required for anyone older than 12. The city also plans to require 5-11 year-olds, approximately 20% of which are currently inoculated in the city, to show proof of their first dose in order to participate in these indoor activities. The Key to NYC mandate already applies to those 12 and up.

Later, at his daily press briefing from City Hall, the mayor added that his administration would set up a phone line that would field questions from business owners. The mandate affects about 184,000 businesses not already covered by existing vaccination rules, according to data shared by the mayor.

The city will share additional guidance on the implementation and enforcement of the new mandate on December 15th, de Blasio said.

At the briefing, he also announced a vaccine mandate for children 5 and up who take part in contact sports or activities that put them at higher risk for COVID-19, such as basketball, ice hockey and singing.

Dr. Denis Nash, an epidemiology professor at City University of New York, applauded the measure, but he cautioned that the city should think of steps beyond vaccination, such as limiting mass gatherings.

“The fact that, during a pandemic, we allow conventions at the Javits Center with over 50,000 people coming from all over the U.S. and the globe, even with delta, is a dereliction of duty on the part of our leaders,” he said, adding that the planned New Year’s Eve ball drop event in Times Square was another example.

The nation's second known case of the omicron variant involved a Minnesota man who attended an anime conference in Manhattan. It was announced Thursday morning and by day's end, New York officials had detected five additional cases and said they suspected community spread to be ongoing. The city and state are already facing increases in COVID cases and hospitalizations due to the delta variant.

The new mandate is likely to face legal challenges. President Joe Biden's order to require all private companies with more than 100 employees to get vaccinated was temporarily blocked last month by a federal appeals court. He has since asked the court to reinstate the order, citing the dangers of the virus spreading in workplaces.

Asked about a possible legal challenge, Georgia Pestana, the head of the New York City’s Law Department who joined the mayor’s briefing via Zoom, said the injunction over Biden’s order stemmed from questions around the authority of the U.S. Occupational Safety and Health Administration, which issued the emergency rule last month, as well as the Centers for Medicare and Medicaid Services, which ordered a mandate in all health facilities.

“I don’t believe there is a question that Dr. Chokshi has the authority to issue this mandate,” Pestana said. She added that the “across the board nature” of the mandate, which now extends to all businesses with in-person workers, “makes it defensible.”

Alicia Ouellette, the dean and president of Albany Law School, said the city would need to answer questions about how religious and medical exemptions would be handled. She also said the city wouldn’t necessarily need to enforce the mandate. The new rule could just be based on the honor system.

“There are lots of health and safety regulations that are placed on employers, and employers are expected to comply, whether they have to report out affirmatively how they've complied,” Ouellette said. “It depends on the particular regulation.”

Kathryn Wylde, the president of the Partnership for New York -- a group that represents the city’s business sector, said she was blindsided by the news, adding that she only learned of the new mandate through news reports.

Wylde said that she was confused as to how the city could issue such an order when Biden’s order was stalled in court. “A mayoral mandate that expects employers to be the enforcement agent does put employers in a difficult position,” she argued.

She added that the new order would only place “more stress, more tension” around the ongoing effort by companies to have their employees resume in-person work.

Andrew Rigie, the executive director of the NYC Hospitality Alliance, a group that represents the restaurant industry, objected to the new requirement that children ages 5 and up be vaccinated. Citing the needed revenues from tourists, he said in a statement: “Given the rapidly approaching holidays and considerable impact of the December 14 deadline, the proposal should be delayed until next year.”

According to a recent survey by the Partnership for New York, only about half of the companies in Manhattan have a vaccine mandate as part of their return to work policies, while another 39% use vaccine-or-test policies for their in-person employees. About one in ten companies -- 11% -- have no COVID-19 vaccine policy.

The head of Staten Island's Chamber of Commerce voiced some opposition to the new requirements, citing a need for flexible solutions for businesses.

“Weeks before the holidays, and with only a few weeks left as Mayor de Blasio’s latest announcement to require private-sector employees to be vaccinated continues to cause angst in the local business community," said Linda Baran, president & CEO of the Staten Island Chamber of Commerce. "Attracting new hires and retaining employees continues to be a pressing issue, and this mandate will only exasperate the problem."

De Blasio said he informed New York Gov. Kathy Hochul of his decision on Monday morning, though he did not indicate what her response was. “I told her why it was the right thing for us to do now,” he said. “We had a good conversation.”

The mayor said he consulted with Adams before he left for Ghana last Monday about his “initial thinking” and once again on Friday night as his administration was “perfecting” the proposal. “I’m sure he’ll be speaking to it,” he said, adding that Adams has consistently expressed his own urgency around the pandemic.

Adams is not expected to return to New York City until December 8th.

Evan Thies, a spokesman for Adams, issued the following statement: “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.”

This story has been updated.