United Airlines will require employees to be vaccinated against the coronavirus, the company announced Friday, becoming the first domestic airline to require the vaccine as a condition of employment.

The company’s mandate will apply to all 67,000 of its active, U.S.-based employees, the company said.

“We know some of you will disagree with this decision to require the vaccine for all United employees,” Scott Kirby, United’s chief executive, and Brett Hart, the company’s president, 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.”

The airline’s announcement comes amid a surge in coronavirus cases nationwide, fueled by the fast-spreading delta variant. The spike in cases spurred U.S. health officials to revise recommendations on mask-wearing for those who have been fully vaccinated and has prompted several major companies — including Google, Facebook and Walt Disney Co. — to require that employees be vaccinated.

United’s mandate, however, makes it an outlier among U.S. carriers, which have largely relied on incentives, such as extra vacation days or gift cards, to encourage employees to get inoculated. Delta Air Lines announced a vaccine mandate in May that applies only to new employees.

Hours after United’s announcement, Frontier Airlines said Friday that it established a policy for employees to be fully vaccinated by Oct. 1. It said employees “that choose not to or are unable to get vaccinated” will be asked to provide negative coronavirus test results on a regular basis.

Alaska Airlines this week began requiring employees to tell the company whether they have been vaccinated.

It’s not clear whether other airlines will follow United’s approach, but at least one carrier ruled it out this week.

The hyper-transmissible delta variant of the coronavirus has left would-be travelers uncertain. The Post spoke to an expert about how to safely make that call. (The Washington Post)

American Airlines chief executive Doug Parker said Thursday on a New York Times podcast that he hoped $50 gift cards and an extra vacation day in 2022 to employees who are vaccinated by Aug. 31 would be enough of an incentive.

“We certainly encourage it everywhere we can, encourage it for our customers and our employees, but we’re not putting mandates in place,” he said.

Southwest Airlines said it “strongly encourages” employees to be vaccinated.

At United, Kirby had signaled in January that the company would consider a vaccine mandate.

In making their case to employees, Kirby and Hart said a growing body of data about the effectiveness of the vaccines convinced them that a mandate is the right decision. They noted that more than 50 medical groups support vaccine requirements, including the American Medical Association, the American Academy of Pediatrics and the American Public Health Association.

“Over the last 16 months, Scott has sent dozens of condolence letters to the family members of United employees who have died from covid-19,” they wrote. “We’re determined to do everything we can to try to keep another United family from receiving that letter.”

Under United’s program, employees will be required to show proof of vaccination five weeks after the Food and Drug Administration announces full approval for a coronavirus vaccine, or five weeks after Sept. 20, whichever is first. Based on that timeline, the latest possible deadline for meeting the requirement would be Oct. 25, the airline said.

Both Pfizer and Moderna have applied for full approval of their two-shot vaccines. Exemptions to the mandate will be offered for United employees who have religious concerns or medical conditions, the company said. Eighty percent of United flight attendants are already vaccinated, according to the Association of Flight Attendants-CWA union.

Employees who show proof of vaccination by Sept. 20 will receive an additional day of pay.

Alaska Airlines said its “attestation” process will give the company a baseline understanding of vaccination rates.

“This data will allow us to make informed decisions on the additional safety protocols we need to have in place to keep our employees and guests safe,” the company said. While the company does not currently plan to mandate vaccines, “we are exploring other ways to keep employees safe, such as mandatory weekly COVID testing for employees who choose not to get vaccinated,” the carrier said.

Frontier president and chief executive Barry Biffle said the rapid rise of the delta variant raises his concerns for the well-being of company employees, as well as their families and friends.

“We need to take every step possible for us to keep our teams safe, protect the operation and protect our passengers,” he said. “The time has come to do what we can to help put an end to COVID-19.”

Frontier said if employees decline to get vaccinated, they “can choose to test regularly as a condition of employment.” Airline and union leaders should jointly “establish testing protocols that work for employees, their health” and overall safety, it said.