United Airlines will require its 67,000 U.S. employees to get vaccinated against Covid by no later than October 25 or risk termination, a first for major U.S. carriers that will likely ramp up pressure on rivals.

Airlines including United have so far resisted vaccine mandates for all workers, instead offering incentives like extra pay or time off to get inoculated. Delta Air Lines in May started requiring newly hired employees to show proof of vaccination. United followed suit in June.

United’s requirement is one of the strictest vaccine mandates from a U.S. company and one that includes employees who interact regularly with customers like flight attendants and gate agents.

U.S. companies such as Facebook announced employees have to prove that they have been vaccinated to return to the office. Others are requiring them for only for certain workers. Walmart, for example, said last week that it will be required for corporate staff and management-level employees. Uber said U.S. office staff will need to be vaccinated to return to in-person work but stopped short of requiring them for drivers.

Meatpacker Tyson Foods said this week its 120,000 U.S. employees must be fully vaccinated this year, though more than 50,000 already are.

“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 Friday in an employee note. “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 said in January he wanted to make Covid vaccines mandatory and that other companies should do the same.

Ending the Covid-19 pandemic is especially crucial for airlines, among the hardest-hit industries from the pandemic. While summer vacation bookings surpassed what most executives were expecting, the fast-spreading delta variant is starting to weigh on demand.

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

United Airlines employees must upload proof that they received two doses of Pfizer or Moderna vaccines or one dose of Johnson & Johnson’s single dose five weeks after federal officials give full approval to them or by Oct. 25, whichever is first, the executives said. Exceptions will be made for certain health issues or religious reasons, United said.

The mandate does not apply to regional airlines that fly 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, according to company officials. United didn’t disclose the company’s overall vaccination rate.

In comparison, about 60% of American Airlines’ pilots are vaccinated, according to an Aug. 5 letter to members from their union, the Allied Pilots Association, which encouraged aviators to get vaccinated.

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

The decision was partly driven by concerns about rises in Covid-19 cases last year during the fall and winter, company officials said.

The Air Line Pilots Association, which represents United’s more than 12,000 aviators, believes the mandate is legal. It said the “small number of pilots” who don’t agree with the policy or plan to remain unvaccinated should contact their chief pilot’s office.

Flight attendants’ labor union, the Association of Flight Attendants, urged cabin crew members to get vaccinated, after United’s announcement.

“COVID-19 is a threat,” the union told members. “There are proven strategies to mitigate that threat. Vaccination is necessary to end the pandemic and the health and economic harm it has caused.”

American Airlines, for its part, said it hasn’t changed its policy that encourages but doesn’t mandate vaccines for employees. Delta Air Lines and Southwest Airlines didn’t immediately say whether they plan to mandate vaccines as well.