United Airlines will require its 67,000 US employees to be vaccinated against Covid by October 25th or risk being fired, a first for major US airlines that is likely to put pressure on rivals.

Airlines, including United, have so far opposed vaccine mandates for all workers and instead offered incentives such as additional pay or time off for vaccination. Delta Air Lines started asking newly hired employees to provide proof of vaccination in May. United followed suit in June.

United’s requirement is one of the strictest vaccination regulations of any US company and involves employees who regularly interact with customers such as flight attendants and gate agents.

US companies like Facebook announced that employees would need to prove they were vaccinated in order to return to the office. Others only ask for it for certain employees. Walmart, for example, said last week that it will be required for company employees, but not warehouse or warehouse workers. Uber said US office staff must be vaccinated to return to personal work, but it was no longer needed to be used by drivers.

Meatpacker Tyson Foods said this week that its 120,000 U.S. employees will need to be fully vaccinated this year, when that is already more than 50,000.

“We know some of you will disagree with this decision to require the vaccine for all United employees,” United CEO Scott Kirby and President Brett Hart said in an employee note Friday. “But we have no greater responsibility to you and your colleagues than to ensure your safety at work, and the facts are crystal clear: everyone is safer when everyone is vaccinated.”

Kirby said in January he wanted to make Covid vaccines mandatory and other companies should do the same.

Ending the Covid-19 pandemic is especially critical for airlines, which are among the industries hardest hit by the pandemic. While summer vacation bookings exceeded the expectations of most executives, the rapidly expanding delta variant is weighing on demand.

“In the past 16 months, Scott has sent dozens of letters of condolence to the family members of United employees who have died of COVID-19,” executives said. “We are determined to do everything in our power to prevent any other United family from receiving this letter.”

United Airlines employees must upload proof that they have received two doses of Pfizer or Moderna vaccine, or a single dose of Johnson & Johnson’s vaccine, five weeks after full approval by federal officials or by October 25, whichever comes first have received. Exceptions are made for specific health or religious reasons, United said.

The mandate does not apply to regional airlines flying shorter routes for United.

Many of United’s employees have already reported they have been vaccinated, including more than 90% of pilots and 80% of flight attendants, company officials said. United did not disclose the company’s overall vaccination rate.

By comparison, according to an August 5 letter to members of their union, the Allied Pilots Association, about 60% of American Airlines pilots are vaccinated, encouraging Airmen to get vaccinated.

United didn’t say what the company’s overall vaccination rate is.

The decision was driven in part by concerns about a spike in Covid-19 cases over the past fall and winter, company officials said.

The Air Line Pilots Association, which represents United’s more than 12,000 airmen, believes the mandate is legal. It said the “small number of pilots” who disagree with the policy or plan to remain unvaccinated should contact their chief pilot’s office.

The flight attendants’ union, the Association of Flight Attendants, urged cabin crew to get vaccinated following United’s announcement.

“COVID-19 is a threat,” the union told members. “There are tried and tested strategies to mitigate this threat. Vaccinations are necessary to end the pandemic and the health and economic damage it has caused.”

For its part, American Airlines said it has not changed its policy of promoting, but not mandating, vaccines for employees. Delta Air Lines said it “strongly” encourages workers to get vaccinated, but does not require all employees to do so, except for new hires. More than 73% of the approximately 75,000 employees are vaccinated, it said.

Southwest Airlines didn’t immediately say whether it plans to prescribe vaccines.