United Airlines will require employees in the U.S. to be vaccinated against COVID-19 by late October, perhaps sooner, joining a rising number of big corporations that are responding to a surge in virus cases.

United was the first major U.S. airline to announce such a move. A smaller carrier, Frontier Airlines, said later Friday that it will require employees to be fully vaccinated by Oct. 1 or face “regular” testing for the virus.

Other airlines have offered extra pay or time off to employees who get vaccinated, but have not required them to get the shots.

United officials called their decision a matter of safety and cited “incredibly compelling” evidence of the effectiveness of the vaccines.

“We know some of you will disagree with this decision to require the vaccine for all United employees,” CEO Scott Kirby and President Brett Hart told employees Friday. But, they added, “the facts are crystal clear: everyone is safer when everyone is vaccinated.”

United, which has 67,000 employees in the United States, has required new hires to be vaccinated since mid-June. Unvaccinated workers are required to wear face masks at company offices.

The Chicago-based airline estimates that up to 90% of its pilots and close to 80% of its flight attendants are already vaccinated. They get incentives to do so.

A United executive said the airline has no plans to require that passengers be vaccinated, calling that a government decision. And the mandate won’t extend to employees at smaller airlines that operate United Express flights.

United told U.S. employees Friday that they will need to be fully vaccinated by Oct. 25 or five weeks after the Food and Drug Administration grants full approval to one of the vaccines — whichever date comes first. The FDA has only granted emergency-use approval of the Pfizer, Moderna and Johnson & Johnson vaccines, but the agency is expected to soon give full approval to Pfizer, according to published reports.

Each employee will have to send an image of their vaccine card to the company. Those who don’t will be terminated, with exemptions granted only for employees who document religious or health reasons for not getting the shots, officials said.

Employees who are already vaccinated or do so by Sept. 20 will get an extra day of pay, according to the memo from Kirby and Hart.

The Air Line Pilots Association said in a note to members that a “small number of pilots” don’t agree with this new policy, but the union believes it is legal. The Association of Flight Attendants encouraged members to get vaccinated and said United’s announcement is not surprising because Kirby has spoken in favor of a mandate for several months.

United’s closest rivals — American Airlines, Delta Air Lines and Southwest Airlines — have not required vaccinations, company representatives said Friday.

American CEO Doug Parker told The New York Times this week that he favors incentives, “but we’re not putting mandates in place.” Delta CEO Ed Bastian told CNBC it would be difficult to require employees or domestic travelers to vaccinate until the vaccines receive full FDA approval.

At Delta, which also requires new hires to be vaccinated, 73% of the workforce has received the shots, according to Bastian.

Airlines and other companies in the travel business have been hit particularly hard by the pandemic, which led to sharp travel restrictions.

The United States requires people entering the country, including U.S. citizens, to show proof of a negative COVID-19 test, and the Biden administration plans to require non-U.S. citizens to be vaccinated before entering the country. Some countries require visitors to be vaccinated or test free of the virus to avoid quarantines.

Since the highly contagious delta variant became the leading strain of COVID-19 in the U.S., most airlines have said it hasn’t affected ticket sales. However, Frontier Airlines executives on Wednesday blamed the variant for weaker bookings in the past week.

Before the United announcement, Disney and Walmart announced vaccine mandates for white-collar workers, and Microsoft, Google and Facebook said they will require proof of vaccination for employees and visitors to their U.S. offices.

This week, Tyson Foodsannounced it will require all U.S. employees to get vaccinated by November — notable because unlike the tech companies, Tyson relies on many lower-paid workers who cannot do their jobs remotely.

A few governments are getting involved. California and New York City will require employees to be vaccinated or face weekly testing, and the California mandate extends to workers in public and private hospitals and nursing homes.

Further, some big companies including Amazon have delayed reopening offices, which will likely push back any significant recovery in lucrative business travel. Starting Monday, Amazon will be requiring all of its 900,000 U.S. warehouse workers to wear masks indoors, regardless of their vaccination status.

The new policies come as the U.S. struggles with a surge in infections. The seven-day average of new reported coronavirus cases has jumped to more than 90,000 a day from around 12,000 a month ago, although hospitalizations and deaths have risen more slowly.

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David Koenig can be reached at www.twitter.com/airlinewriter