New York City is initiating a stricter COVID-19 vaccine mandate that will go into effect on Dec. 27. On Monday, Mayor Bill de Blasio announced the new policy and described it as a “preemptive strike” to stall another wave of COVID cases after the Omicron variant was detected in the five boroughs last week.

In an interview with MSNBC, he described the move as a necessity at a time when the conditions for a surge in cases are dire.

“We in New York City have decided to use a preemptive strike to really do something bold to stop the further growth of COVID and the dangers it’s causing to all of us," de Blasio said on "Morning Joe."

“At this point, Omicron is here. It looks like it's very transmissible. That's just going to make a tough situation even harder,” he continued, adding that the timing of Omicron’s emergence was “horrible” given the arrival of winter. 

Under the new policy, vaccination requirements to visit certain businesses are going to tighten considerably. Instead of the previous proof of one dose of a COVID vaccine to enter places like gyms or restaurants, New Yorkers will have to show they are fully vaccinated. 

The updated mandate will also apply to children ages 5 to 11, but they will only need proof that they received a single dose. This group received approval to receive COVID-19 vaccinations last month, but de Blasio said that it was his hope that parents would see this mandate as an impetus to get their kids vaccinated. 

"Our youngest kids, we got to reach them now. Right now in this city, about 20% have gotten to that stage in that 5 to 11 range. The vaccine is relatively new," he said. "But what we're trying to say to parents is that it's urgent. Before Omicron grows, before Delta continues to stress us even worse in the winter months, get your kid vaccinated. And here's an incentive to do it."

New York City, once the epicenter of COVID-19 during the early months of the pandemic, has made significant progress in vaccinating its population. According to the most recent data, 81.6% of adults in the city are fully vaccinated and 89.2% have received at least one dose.

The Omicron variant has appeared in the city. Last week, New York Gov. Kathy Hochul confirmed that five cases were detected in the state, including four spread out across the city.