United Airlines announced Friday that employees will be mandated to get a COVID-19 vaccine, becoming the first major U.S.-based carrier to announce such a requirement.

The strict policy details that employees must be vaccinated five weeks after the Food and Drug Administration fully approves a COVID-19 vaccine or five weeks after Sept. 20, whichever comes first.

Denver is a hub city for United, and more passengers fly out of Denver International Airport by volume on United than any other carrier. Through July, nearly 10.51 million passengers have flown with United via DIA in 2021, representing 42.5% of travelers using the airport .

“We know some of you will disagree with this decision to require the vaccine for all United employees,” United CEO Scott Kirby and President Brett Hart wrote in a letter to employees. “But, we have no greater responsibility to you and your colleagues than to ensure your safety when you’re at work, and the facts are crystal clear: everyone is safer when everyone is vaccinated.”

United is offering an extra day of pay to employees who will be inoculated before the deadline, including those who have already gotten the vaccine.

The policy does not apply to Untied’s passengers.