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NEW YORK — As concerns over COVID-19 variants grow, Mayor Bill de Blasio on Monday announced a first-in-the-nation vaccine mandate for private sector workers across New York City, expanded the Key to NYC vaccine requirement to include children as young as 5 and said the pass will soon require a second dose.

De Blasio announced the updates in an interview on MSNBC’s “Morning Joe.”

“Our health commissioner will announce a vaccine mandate for private sector employers across the board,” the mayor said.

De Blasio and health commissioner Dr. Dave Chokshi were set to speak at his COVID briefing at 10 a.m. Monday.

The vaccine mandate will apply to about 184,000 businesses, according to the mayor’s office. It is set to go into effect on Dec. 27, just days before mayor-elect Eric Adams takes office.

Between this new mandate and the prior mandate for all city workers, just about anyone who works in New York City will be mandated to get the shot in order to go to work.

Additionally, the city is expanding its Key to NYC vaccine mandate for city businesses, indoor dining, gyms, theaters and other entertainment venues to include children ages 5 to 11.

The mayor’s office said approximately 20% of those kids have gotten at least one shot of the vaccine.

Parents will have to show proof of vaccination for those young children starting Dec. 14.

“I urge parents really strongly, get that vaccination. It’s safe. It’s been proven. Here’s another incentive to do it,” the mayor said.

De Blasio also announced that, beginning Dec. 14, city kids 5 to 11 will be required to get vaccinated to participate in high-risk extracurricular activities like sports, band, orchestra and dance.

Key to NYC is also expanding to require proof of a second vaccine dose for all those 12 and older, except those who have received the Johnson & Johnson vaccine, de Blasio said. The initial mandate only required proof of a first vaccine dose to gain entry.

“We’ve been living with this now for most of two years,” de Blasio said. “We got to put it behind us and vaccine mandates in my experience are the one thing that really breaks through.”

When asked how the city will enforce the widespread mandate, the mayor said his office is working to put together rules now and will work with the business community.

“We’ve seen a lot of cooperation so far when we put in place our mandate for example, for restaurants, indoor entertainment, indoor fitness, we actually got a lot of cooperation,” he said.

Dr. Ted Long, executive director of New York City’s Test and Trace Corps., told the PIX11 Morning News on Monday that the new mandate for private sector workers and expanded Key to NYC requirements will help strengthen New York City and its ability to slow the spread of the virus.

“In New York City we’ve led the way for the country in terms of getting people vaccinated through mandates,” Long said. “It’s been a big credit to how we’ve been successful in New York City.”

Long pointed to the anime conference at the Javits Center as an example of how vaccination requirements work to protect New Yorkers, after a person who later tested positive for the new omicron variant attended the event.

“We’re still investigating it, but we haven’t seen evidence of widespread transmission. And if, indeed at the end of this, we still don’t see any evidence of widespread transmission, the fact that we required everybody in New York City to be vaccinated in order to go to that conference definitely helped substantially,” he said.