United Airlines will require all 67,000 of its U.S.-based employees to become fully vaccinated by Oct. 25 or risk losing their jobs, becoming the first major American airline to do so.

CEO Scott Kirby made the announcement in a letter to employees Friday in which he said, "The facts are crystal clear: everyone is safer when everyone is vaccinated."

Airlines, including United, thus far have resisted mandating vaccines, opting instead to incentivize vaccination through bonuses or extra time off, NBC News reported. But now that has changed. The outlet added that United's decision will likely ramp up pressure on competitors.

"We know now that an unvaccinated person is about 50 times more likely to be hospitalized for COVID-19 than a vaccinated person and nearly 300 times more likely to die," Kirby wrote in the letter.

Then he handed down the specific instructions: "All U.S.-based United employees will be required to upload a vaccine card showing you have received two doses of the Pfizer or Moderna vaccine (or one dose of the J&J vaccine) five weeks after the FDA has announced it has fully approved a COVID-19 vaccine or five weeks after September 20th, whichever comes first."

The chief executive acknowledged that his decision would not be popular among all employees, but argued that protecting employees in this way was part of the company's "greater responsibility."

"We know some of you will disagree with this decision to require the vaccine for all United 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," he said.

Kirby has wanted to mandate vaccines since the beginning of the year but felt it was logistically impossible and too controversial at the time.

He told employees in January that he had "confidence in the safety of the vaccine" and believed requiring vaccination among employees was "the right thing to do." But he said he didn't think United would "get away with" or "realistically be" the only company to implement a vaccine mandate.

Things have shifted since then. NBC News noted that other U.S. companies such as Facebook, Walmart, and Uber have implemented various vaccine requirements. The outlet said United's is "one of the strictest vaccine mandates from a U.S. company."

With the new mandate, United joins Tyson Foods. The meatpacker also said this week that all of its U.S. employees — 120,000 of them — must be fully vaccinated by the end of the year.