New York City is imposing a vaccine mandate for all private sector employers as a preemptive measure to fight a surge of Covid cases this winter, Mayor Bill de Blasio announced on Monday.

The mandate covers 184,000 businesses and will go into effect on Dec. 27, de Blasio said. Everyone 12 and older will be required to show proof of two vaccine doses by that date, unless they received Johnson & Johnson’s single-dose vaccine.

“We’ve got omicron as a new factor, we’ve got the colder weather, which is really going to create additional challenges with the delta variant, we’ve got holiday gatherings,” de Blasio told MSNBC Monday morning. “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.”

The city will also require proof of vaccination for children ages 5 to 11 for indoor dining, entertainment and fitness establishments, the mayor said. Kids will be required to have proof of at least one does by Dec. 14. They will also have to show proof of one dose for high-risk extracurricular activities such as sports, band, orchestra and dance by the same date.

New York has identified eight cases of the omicron variant as of the weekend, seven in the New York City area and one in Suffolk County on Long Island.

Covid cases are increasing in New York City, with more than 1,500 new cases confirmed daily on average, according to the city health department.

The New York City policy comes after the Biden administration was forced to halt implementation and enforcement of national vaccine and testing requirements for businesses with 100 or more employees to comply with a federal court order.

Public health officials are worried about a surge in Covid cases this winter from the delta variant as people gather inside to escape the winter chill. They are also concerned that the omicron variant could drive a spike in cases. The World Health Organization said last week that early data suggests omicron is more contagious than past variants, though more information is needed.

It’s unclear whether omicron causes more severe disease, according to the WHO. The people who have tested positive for omicron in the U.S. so far have shown mild to moderate symptoms.