United Airlines will require all of its U.S.-based employees get the COVID-19 vaccine by this fall.

Earlier this year, United CEO Scott Kirby said he wanted to make the vaccine mandatory as long as the Chicago-based airline wasn’t the only company to do so. Now, with COVID-19 cases rising, United joins companies including Google, Microsoft, Facebook and Tyson Foods that have said they will require employees working in person get the shot.

“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,” Kirby and United president Brett Hart said in a letter to employees Friday.

The airline’s 67,000 U.S. employees must provide proof they received all required doses of the Pfizer, Moderna or Johnson & Johnson vaccine by Oct. 25, unless the U.S. Food and Drug Administration fully approves a COVID-19 vaccine before Sept. 20, United said. If a vaccine receives full approval before that date, employees will have five weeks to provide proof they received a shot.

The vaccines have already received emergency use authorization.

Workers who refuse to get the vaccine will be required to leave the company — the same consequence an employee would face for failing to follow a critical safety requirement that is a core part of their job description, United said.

There will be exemptions for employees who decline the vaccine for religious or medical reasons, United said. When it comes to spotting fake vaccine cards, United said it plans to use what it learned from a sweepstakes it held earlier this summer that offered passengers who uploaded vaccine cards a chance to win free flights.

United has already been encouraging employees to get the vaccine, hosting vaccine clinics at airports including O’Hare International Airport and offering pilots extra pay and flight attendants additional vacation days as incentives to get the shot. Employees who haven’t already received an incentive will get an extra day’s pay if they provide proof of vaccination before Sept. 20, United said.

It’s not clear how many of United’s employees have already received the vaccine, but in groups that received incentives to provide proof of vaccination, most already got the shot, including roughly 90% of pilots.

The Air Line Pilots Association, which represents United’s pilots, acknowledged there is a “small number” who do not agree with the company policy but said it believes the mandate is legal in a message members of the union.

Nearly 80% of United’s flight attendants have also received the vaccine. In a message to union members, the Association of Flight Attendants said it has seen a “notable uptick” in positive COVID-19 tests over the past two weeks, mostly among unvaccinated flight attendants.

“With all that we have been through since the beginning of the pandemic, all that we have fought for and won, there is now too much at risk to not ensure the safety and well-being of United Flight Attendants … Vaccination is necessary to end the pandemic and the health and economic harm it has caused,” the Association of Flight Attendants said in a message to members.

Vaccines are also playing a role in bringing corporate employees back to offices, including the airline’s Willis Tower headquarters. There is no company-wide policy on where employees should be working during the pandemic — instead, it’s up to each team, United said.

People who work at an office must upload proof of vaccination if they want to work without a mask. The airline has an app that will show a green check mark verifying the employee has been vaccinated and encourages employees to display the app when meeting with others in the office.

In response to recent guidance from health officials advising everyone, even those who are vaccinated, to wear masks indoors in areas with “substantial” or “high” transmission, the airline is encouraging people to wear masks indoors if they can’t socially distance or live with someone who isn’t vaccinated.

Other major U.S. airlines have not issued companywide vaccine mandates, though Delta Air Lines, like United, requires new hires be vaccinated.

The airline told employees Kirby has sent dozens of condolences letters to family members of employees who died of COVID-19.

“We’re determined to do everything we can to try to keep another United family from receiving that letter,” the airline said.