In order to attend large indoor, unseated events in the Denver metro area, attendees will soon have to be fully vaccinated against COVID-19, according to a public health order published Sunday.

The vaccine requirement will go into effect Friday and apply to all public, unseated indoor events of 500 attendees or more in Denver, Arapahoe, Adams, Boulder, Broomfield and Jefferson counties. The vaccine mandate is currently set to expire Dec. 31.

The order is an attempt to boost vaccinations and avoid super-spreader events, the state health department said.

“I am grateful for the counties that are coordinating with the state to slow the spread of the virus,” said Jill Hunsaker Ryan, executive director of the health department. “Large venues and local governments are part of the solution to ending the pandemic.”

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Under the order, all attendees aged 12 and older must be fully vaccinated to be admitted to large events. In place of full vaccination, events may accept negative COVID-19 tests until Dec. 1. Afterward, only proof of vaccination will be accepted.

The mandate makes Colorado one of a handful of places in the country to compel private businesses to offer services only to certain customers based on health choices. The state of Washington and Los Angeles County have implemented similar vaccine mandates for large events in recent weeks. 

This order comes as Colorado is nearing its worst COVID-19 hospitalization peak of the pandemic.

On Wednesday, state epidemiologist Rachel Herlihy said Colorado could surpass its hospital capacity by the end of the year, with the state projected to hit 2,258 hospitalizations by Jan. 1, compared to the 1,841-patient peak in 2020.

Currently, there are about 1,500 COVID-19 patients hospitalized in Colorado and nearly 95% of ICU beds are occupied. Of those patient s, 81% are unvaccinated.

Gov. Jared Polis announced plans to require vaccines at large events on Wednesday. While the requirement could expand statewide, the order published Sunday only “strongly encourages” vaccine mandates for large events outside of the Denver metro area.

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“The most important thing you can do is get vaccinated,” Polis said Sunday during an appearance on CBS’ “Face the Nation.” “Right now, if you’re vaccinated, your risk is one-tenth or one-twelfth what it was during the highest peak before. … If you’re unvaccinated, this is the most dangerous time for you.”

The health department said the event vaccine requirement is part of the state’s commitment to getting through the COVID-19 surge without implementing statewide closures or capacity restrictions, in order to minimize economic impacts.

Polis issued an executive order Thursday to allow all adult Coloradans to receive the COVID-19 booster shots, going against federal guidance to limit the shots to high-risk individuals. In the order, Polis said all of Colorado is at high risk of infection.

As of Friday, 79.56% of eligible Coloradans had received at least one dose of the COVID-19 vaccine, according to state data.