As cases climb again in the US, New York City Mayor Bill de Blasio made an announcement on Monday that all private employers will have to require their workers to get vaccinated against COVID-19.

The mayor’s move is considered one of the most aggressive vaccine rules in the nation and came following the Omicron variant's worrying spread in New York and other countries in the world.

“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,” he said.

The mandate will be effective as of December 27, as per De Blasio, and workers will need to provide proof of having received at least one dose of the vaccine.

The decision will apply to roughly 184,000 businesses in the city of 8.8 million people, according to a spokesperson for the mayor.

De Blasio said the measure aims to prevent a spike of infections amid holiday gatherings and as cold weather drives more people indoors, where the virus is more likely to spread.

While vaccine rules across states and cities vary widely, no state has announced a broad private-sector mandate as New York City has, according to the nonpartisan National Academy for State Health Policy.

The mayor added that any person aged 12 or older who wants to dine indoors at a restaurant, go to a gym or see a show will have to show proof of having received two shots of the vaccine. As for children aged 5 to 11, they will have to show proof of at least one shot.

US President Joe Biden sought to impose a similar mandate nationwide, one that would apply to businesses with 100 or more workers, but federal courts have put that on hold ahead of the January 4 deadline. 

Cases of the Omicron variant have been reported in about one-third of the states, but scientists are not sure yet whether it is more dangerous than previous versions. The Delta variant still accounts for nearly all infections in the US.