This story has been updated.

There is no requirement for air travelers within the United States to be vaccinated against covid-19. But depending on which airline you fly, there might be a mandate for employees.

While all major U.S. carriers are trying to get their workforces vaccinated, some are going further than others, with United being the first to mandate vaccinations, in August. Other airlines instead have been offering their employees incentives, mandating testing for the unvaccinated or tacking on an extra insurance charge.

But President Biden’s order last month — which said federal employees and contractors must be vaccinated with no option to test instead and that companies with 100 or more employees must mandate vaccination or weekly testing — is changing the game.

“If indeed the mandate now is everyone must be vaccinated or ... tested once a week, we will obviously comply by that mandate,” Doug Parker, chief executive of American Airlines, said in a Washington Post Live interview in September. “All along, as we’ve been going through this, we have been considering mandates and may have done one on our own. But what we wanted to do was do everything we could first to encourage everyone to do so.”

On Friday, a handful of airlines — including American — said they would start mandating vaccinations because the work they do with the government classifies them as federal contractors. Reuters reported that the White House has urged American, Delta and Southwest to require employees to be vaccinated by Dec. 8.

Here’s what American Airlines and other major U.S. airlines are requiring for employees.

Alaska Airlines

The airline confirmed Monday that its employees fall under the vaccine mandate for federal contractors. That means, according to a statement, that all workers — “including certain contractors and vendors” — will either need to be vaccinated or approved for an exemption.

“The date by which employees must be fully vaccinated has not been confirmed by the government, but it could be as early as Dec. 8, 2021,” the statement said. “That’s why we encourage all of our employees to begin the vaccination process as soon as possible, and to allow time for their vaccination regime to be fully completed.”

Even before the update, all new employees at Alaska Airlines and its regional carrier, Horizon Air, needed to be vaccinated. For those already employed, the airline is offering $200 to workers who show proof of vaccination; those who aren’t vaccinated face a slew of requirements. The airline has extended the deadline from Oct. 15 to Dec. 1 to give vaccinated employees the money.

Previously, Alaska Airlines said it would “implement a testing protocol for unvaccinated employees as another layer of safety, while continuing to enforce safety protocols such as masking and distancing,” spokesperson Ray Lane said in an email.

Of the Alaska and Horizon employees who have shared their status, about 75 percent are vaccinated, the company said last month.

“We believe having as many people as possible vaccinated is the best path for protection against covid-19, and we will continue to strongly encourage our employees to be vaccinated,” Lane said.

American Airlines

In a memo following Biden’s vaccine-or-test order, Parker and American Airlines President Robert Isom told employees that the company would await details before determining how to proceed. But the executives urged workers to get vaccinated “sooner rather than later.”

A follow-up memo on Friday said the government had provided those details, and all American U.S.-based employees — as well as some international crew members — would have to be vaccinated with no option to choose testing instead.

“While we are still working through the details of the federal requirements, it is clear that team members who choose to remain unvaccinated will not be able to work at American Airlines,” the memo said. Workers can request accommodations if they have a disability or a religious reason that would exempt them from a vaccination.

American offered incentives, such as an extra day of vacation pay this year and a $50 gift card, to those who got the shot and submitted proof by Oct. 1. Reuters reported last month that unvaccinated employees at American will not be eligible for special leave if they have to quarantine because of the virus; they would have to use their own sick time or leave.

Parker said last month that he didn’t have an exact number of employees who were vaccinated, but he did not believe it is 100 percent of the workforce. While he said the initial approach was a carrot rather than a stick, he acknowledged that tougher measures were probably on the way.

“Up to this point, we haven’t put people in that position of having to choose whether or not they are vaccinated or employed,” he said. “That’s coming, though.”

Delta Air Lines

Vaccination has been a requirement for Delta hires since May. And since Sept. 12, its unvaccinated U.S. employees have had to take a coronavirus test every week while community case rates are high, chief executive Ed Bastian announced. Those employees also will be subject to a $200 monthly surcharge on their health-care plans starting Nov. 1.

“This surcharge will be necessary to address the financial risk the decision to not vaccinate is creating for our company,” Bastian wrote.

As of Sept. 30, coronavirus pay protection will be provided only to vaccinated workers who get a breakthrough infection, the memo said.

Spokesperson Morgan Durrant said in an email that there had been no resignations as a result of the policies, and the workforce vaccination rate had increased from about 75 percent to 82 percent as of last month. The company also has offered incentives such as paid time off and cash giveaways.

On Monday, Durrant said the airline is evaluating the administration’s plan. So far, 84 percent of employees have been vaccinated.

Frontier Airlines

The low-cost carrier said in August that all direct employees must be fully vaccinated by Oct. 1. Those who choose not to get the shot, or can’t, will need to show proof of a negative coronavirus test “on a regular basis.”

“The good news is that the vast majority of our employees have already taken this important step and have gotten vaccinated,” chief executive Barry Biffle said in a news release.

Hawaiian Airlines

U.S.-based employees must be vaccinated by Nov. 1, the Honolulu Star-Advertiser reported this past month. Those who are approved for medical and religious exemptions will be required to have regular tests, the paper said. Hawaiian did not respond to questions seeking more details.

JetBlue Airways

Executives told workers last week that federal government mandates would require every U.S. crew member — even if they work at home or in a support center — to be vaccinated. The only exceptions are for medical and religious accommodations, with no testing option.

“We’re still working through what the deadline may be but it could be as early as December 8, 2021 and so we need to plan on that basis,” chief executive Robin Hayes and president and chief operating officer Joanna Geraghty said in a memo.

The carrier said complying with the mandate was “not a choice” and provided a timeline for when employees should get their first dose, depending on which vaccine they choose, and reiterated the importance of complying before the holidays.

“With the possible federal deadline for vaccination just before the December peak season, it’s important that you schedule your shots now so that you will be able to continue working,” the memo said. “Our customers count on us to get them where they’re going during the holidays, and we need to be ready to fully comply with the mandate before the holiday peak starts and to help bring this pandemic to a close.”

Southwest Airlines

After a “thorough review” of Biden’s vaccine order, Southwest said Monday that employees must be vaccinated or be approved for an accommodation by Dec. 8 to stay with the company.

“Southwest Airlines must join our industry peers in complying with the federal government’s covid-19 vaccination directive,” said CEO Gary Kelly. “I encourage all Southwest employees to meet the federal directive, as quickly as possible, since we value every individual and want to ensure job security for all.”

Previously, the airline said it strongly encouraged employees to get vaccinated. The airline is not testing unvaccinated employees at this point and did not share the percentage of its employees who have been immunized.

In September, the carrier said it would offer about two days of pay to workers who are — or become — fully vaccinated and share proof by Nov. 15.

Starting Nov. 16, employees will have to be vaccinated to be eligible for up to 10 days of quarantine pay if they become infected or are forced to quarantine for work-related exposure.

“As Southwest awaits final details, and the potential application of the Biden administration’s testing and vaccine plan, we continue strongly encouraging employees to receive the covid-19 vaccination and to share their vaccination status with Southwest,” the company said in a statement last month.

United Airlines

All U.S.-based employees at United must be fully vaccinated by Oct. 31, with a deadline of Sept. 27 for at least a first shot, the airline announced last month. And the potential penalties are strict.

“If you have not received and reported at least one shot or have not been granted an accommodation extension, United will start the separation process as early as September 28,” United said in an update to employees.

The carrier said recently that aside from the “small number” of employees who have asked for an exemption, more than 97 percent of its U.S. employees are vaccinated.

“While we continue to be encouraged by the outpouring of support and appreciation that we’ve received from employees, we know the decision to get vaccinated was a difficult one for some,” the airline told employees. “But we also know that everyone is safer when everyone is vaccinated. And vaccine requirements work.”

Workers had to submit an application for an exemption for medical or religious reasons by Aug. 31. Those who were granted exemptions will have to go on temporary leave starting Oct. 2; it’s not clear when that leave will end.

Six employees filed a class-action lawsuit against United over the exemptions, Reuters reported.

“We’re reviewing this complaint in greater detail but at this point, we think it’s without merit,” spokeswoman Leslie Scott said in an email.