United Airlines Holdings Inc. will require its 67,000 U.S. employees to be vaccinated this fall, the first major airline to take this step as the Delta variant drives a nationwide increase in Covid-19 infections.

Airlines including United have so far tried to encourage workers to be vaccinated voluntarily, with incentives such as bonus pay or extra vacation.

In a letter to employees Friday, Chief Executive Scott Kirby and President Brett Hart said they expected that some would disagree with the decision but said the move was necessary to keep workers safe.

“The facts are crystal clear: everyone is safer when everyone is vaccinated,” the executives wrote. “Over the last 16 months, Scott has sent dozens of condolences letters to the family members of United employees who have died from COVID-19. We’re determined to do everything we can to try to keep another United family from receiving that letter.”

Mr. Kirby has been vocal about wanting his entire workforce to be vaccinated but had said earlier this year that such a requirement would be hard to implement if other companies weren’t also requiring vaccines. United has since June required that only newly hired employees be vaccinated, a policy that Delta Air Lines Inc. also implemented.