New York City will implement new measures meant to head off the worsening coronavirus surge, including what its mayor says will be a "first-in-the-nation" vaccine mandate for all private-sector workers to take effect Dec. 27.

Mayor Bill de Blasio announced the new requirements Monday on MSNBC's Morning Joe.

The ag gressiv e new measures also require proof of vaccination with at least one dose for children ages 5-11 for entering in indoor dining, fitness and entertainment venues. Proof of vaccination is already required for people 12 and older to enter those places in New York City.

De Blasio stressed that vaccination was the key to protecting against the virus, including for children.

"Don't end up in a situation where a child is left unprotected. I urge parents really strongly, get that vaccination. It's safe. It's been proven. Here's another incentive to do it," de Blasio said.

The rules also bump up the vaccination level required to be in compliance with other city rules from one dose to two for adults receiving the two-shot mRNA Pfizer-BioNTech or Moderna vaccines.

De Blasio called the moves a "preemptive strike" to stop the spread of the coronavirus. The mayor said more information on the announcement is forthcoming from the city's health commissioner, Dr. Dave A. Chokshi.

Driving the decision, de Blasio said, are concerns over winter weather and holiday gatherings causing more people to gather indoors — where transmission is more likely — and lingering unknowns about how transmissive and dangerous the omicron variant is.

State health officials identified four cases of the omicron variant in the New York City area, New York Governor Kathy Hochul announced Thursday. De Blasio reported community spread is assumed to be happening in the state.

"We knew it would come to New York state at some point," Hochul said. "We're ready for it. This is not surprising."

Scientists are researching the omicron variant — which has already been found in at least 17 states in the U.S. — including the question of how much protection current vaccines may offer against it. Early evidence suggests omicron may be better at evading the immune system than previous strains, but officials say it's too early to tell if it causes a milder or more severe illness than other variants.

The pervasive variant in the United States is still the delta variant, which has been causing about 100,000 cases a day. Case numbers are 50% higher now compared to a month ago, according to the Centers for Disease Control and Prevention.

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