Southwest (LUV  ) announced its company-wide vaccine mandate on Tuesday, joining the likes of JetBlue (JBLU  ), American Airlines (AAL  ), and Alaska Airlines (ALK  ).

The flurry of mandates come after United Airlines (UAL  ) announced that 99% of its workforce was vaccinated. At the time, around 600 employees remained un-exempt and un-vaccinated, with that number now at 232. Should those workers not receive vaccines, United has said that it will terminate them.

Southwest's Tuesday announcement presented a similar requirement to its employees, as did American, Alaska, and JetBlue in their respective announcements previously; receive at least one shot of the COVID-19 vaccine by a deadline or face termination.

"Southwest Airlines must join our industry peers in complying with the federal government's COVID-19 vaccination directive," Southwest CEO Gary Kelly told Employees. "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."

Under President Joe Biden's mandate, all federal employees must be vaccinated by December 8, a rule that applies to contractors as well. Given the litany of federal contracts that major airlines operate, non-compliance isn't really an option.

As a result, airlines are largely ceasing opt-out programs such as masking and testing. While employees may still apply for exemptions on religious and medical grounds, whether or not this will allow them to remain actively at work remains to be seen. United is placing its exempted workers on leave until infection rates decline, for example.

The potential for adjustments and even expansions of the mandate also remains a possibility. Proposals to require vaccination or a negative COVID-19 test for passengers have been pitched in Congress; meanwhile, Dr. Anthony Fauci has floated the idea of applying a similar requirement to foreign travelers. While airlines have balked at such a mandate, the White House has kept an open dialog with industry leaders, as evident by a recent call between an aide and several CEOs, according to Reuters.