United Airlines became the first major airline carrier to require COVID-19 vaccinations for all its workers, in a move announced Friday.

Chief Executive Scott Kirby and President Brett Hart said in a letter to United's 67,000 employees that they must be fully vaccinated by five weeks after the Food and Drug Administration grants full approval to the vaccines, or by Oct. 25, whichever comes first. Till now, the vaccines have been approved under emergency use authorization.

Kirby and Hart acknowledged there will be backlash for the vaccine mandate, but said: "The facts are crystal clear: everyone is safer when everyone is vaccinated."

"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."

United has already been requiring new employees to show proof of vaccination, a rule implemented in June.

As an incentive, the company is offering an extra vacation day to employees who show proof of vaccination by Sept. 20.

Employees who do not show proof of vaccination must leave the company, but the company says it will decide on a case-by-case basis whether to grant exceptions for health or religious reasons.

With cases of the COVID-19 Delta variant surging across the US, a growing number of American companies are now requiring some or all employees to show proof of vaccination, including Google, Facebook, Disney, Walmart and Netflix./aa