Air travel has slowly begun to rebound from 2020, but the path to recovery has been bumpy.
The airlines, which are also government contractors, fall under President Biden's sweeping September order that companies with more than 100 workers require employees to be vaccinated or undergo weekly testing.
The White House has pressed for U.S. airlines to mandate vaccines for staff members by December 8, without the testing option.
To comply with regulations, United Airlines said in September that 97% of its employees had been fully vaccinated – not including a "small number" of employees seeking a medical or religious exemption from vaccination.
Six United employees sued the company, claiming that the airline had been discriminating against employees who get exemptions from the vaccine mandate by placing them on unpaid leave.
At the beginning of the month, American Airlines, Alaska Airlines and JetBlue said they would join United in requiring employees to be vaccinated.
A few days later, Southwest Airlines also told staff they needed to be vaccinated by the December deadline – with approval to skip the shots due to medical or religious reasons – due to the Biden administration's rules.
However, on Thursday, the airline vowed that employees would not be fired for not getting the shot, arguing that it "makes no sense."
While the airline "encourages every employee" to get inoculated, it does not want to "lose any employee" over Biden's federal vaccine mandate, Southwest said in a statement to FOX Business.
Employes must submit a request for exemption from vaccination by Nov. 24.
American has said that workers who are granted medical or religious exemptions will likely have to wear face masks and undergo regular testing – but the carrier is still working on details.
"American will not be placing any team members on unpaid leave as part of the federal vaccine mandate," American spokesman Matt Miller said this week.
Delta Air Lines says it will also let workers undergo regular testing if they don't want to be vaccinated – though they face a $200 monthly surcharge on their health insurance.
Delta CEO Ed Bastian said last week that 90% of the airline's employees are vaccinated and that he expects that figure will reach 95% come November.
United, meanwhile, has begun termination proceedings against around 200 employees who neither got the shots nor asked for an exemption.
What the future holds for passengers during all of this remains unclear, but officials have said that airlines would do their best to make deadlines and not let mandates disrupt holiday travel.
The White House offered a similar message in its COVID-19 briefing on Wednesday.
"Vaccination requirements will not impact holiday travel," White House coronavirus response coordinator Jeff Zients said.
FOX Business' Daniella Genovese and The Associated Press contributed to this report.