NEW YORK (NYTIMES) - Mayor Bill de Blasio announced a sweeping coronavirus vaccine mandate for all private employers in New York City on Monday morning (Dec 7) to combat the spread of the Omicron variant.

Mr de Blasio said the aggressive measure, which takes effect on 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 Covid-19 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 eateries with indoor dining, entertainment outlets and gyms. Nearly 90 per cent of adult New York City residents now have at least one dose of the vaccine.

But Mr de Blasio said the city - once the centre of the pandemic - must go further to combat another wave of the virus. Some private employers have required staff 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 business leaders.

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

Mr de Blasio and Governor 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 Covid-19 cases in the city has increased rapidly in recent weeks; daily case counts have increased more than 75 per cent 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."

Mayor-elect Eric Adams, who takes office on Jan 1, is on vacation in Ghana this week. His spokesman, Mr 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-19 strategies when he is in office and make determinations based on science, efficacy and the advice of health professionals," he said.

Without saying whether she agreed with the measure, Ms Hochul told reporters on Monday afternoon that the mayor had called before his announcement.

"I was aware of this and I support the local government leaders to execute the policies to fight Covid-19... they believe will be most helpful to deal with this pandemic within their own jurisdictions," she said.

Ms Hochul has sought to distinguish herself from her predecessor by empowering local governments to lead the pandemic response in their localities.

"New York City is one of the localities within the state of New York, just as Erie County is," she noted.

The Joe 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 (Osha) issued an emergency rule this month requiring vaccinations for employees of companies with at least 100 workers, although it exempts those who work at home or exclusively outdoors.

Ms Georgia 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 centred on whether Osha had the proper authority.

Ms Kathryn Wylde, president of prominent business group 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, although some policies include testing options, and medical and religious exemptions.

Many questions remain about the Omicron variant. Some early signs suggest 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-19.

Scientists are also waiting to see whether cases lead to substantial hospitalisations and deaths; both are lagging indicators.

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