That means employees would need to get their first dose of the Pfizer vaccine by Nov. 3 and the first dose of Moderna by next week, Oct. 27, 2021. (If they miss those deadlines, they could always opt for a last-minute, one-dose Johnson & Johnson vaccine.)

And employers are starting to feel the heat on these deadlines. “To get an unvaccinated employee to compliance, that’s one of the immediate hurdles that a federal contractor who is subject to the vaccine contractor mandate is facing right now,” says Marc Antonetti, a partner at BakerHostetler focused on labor and employment law.

And there are serious risks for companies that don’t meet that deadline. “If you are not compliant, you could in fact find yourself being terminated as a federal contractor for default,” Antonetti tells Fortune. And that could be a lot of money at stake, depending on the contract and the agency. The federal government spent more than $665 billion on contracts during fiscal year 2020. 

But for many companies, the process will take time, more time than perhaps the current deadline provides. Something Southwest Airlines, which operates as a federal contractor, recently tried to communicate to its workers. 

Recognizing that it may take time to review and validate requests for accommodation, Southwest told employees last week that if the company is not able to evaluate a worker’s request for accommodation by the Dec. 8 deadline, the staff will not be terminated or put on unpaid leave. 

“If a request for accommodation has not been reviewed or approved by Dec. 8, the employee will continue to work, while following all COVID mask and distancing guidelines applicable to their position, until the accommodation has been processed,” the company said in a statement to Fortune on Tuesday. 

“Southwest intends to grant all valid requests for medical and/or religious accommodations. In the event a request is not granted, the company will provide adequate time for an employee to become fully vaccinated while continuing to work and adhering to safety protocols,” the company said. 

However, if an employee has neither been vaccinated nor applied for an accommodation before the deadline, the person does face termination.

“Southwest encourages all employees to receive the COVID-19 vaccination as the airline strives to meet the federal mandate for government contractors,” the company said. 

For employers who may be running into challenges vaccinating their entire workforce, Antonetti says the regulations may provide some carve-outs. “There may be ways to comply without necessarily involving your entire workforce in compliance,” Antonetti says. 

One approach for employers is to segregate their workforce so that only certain departments or employees who are working on a project for the federal government are doing those particular tasks. Those employees would need to be vaccinated, but it may be only part of a company’s overall operations. 

But that could be wide-ranging for some employers. For example, if they have subcontractors or outside legal or HR operations that support the contracted work, then they’ll need to be fully vaccinated under the mandate. 

At the end of the day, it’s going to be a big issue for everyone. “There’s a lot of things to think about on that federal contractor side as to who might actually be covered by the vaccine mandate,” Antonetti says. 

And very little time in which to accomplish it. 

More health care and Big Pharma coverage from Fortune:

• What you need to know about the Delta Plus COVID variant and the danger it poses
• Should kids under 12 get the COVID vaccine? What parents should know
• Intrivo launches new at-home rapid test for COVID amid U.S. supply shortage
• COVID-19 vaccine makers could inoculate the world by the end of 2022, says J&J CEO Alex Gorsky
• Got the J&J vaccine? Here’s what you need to know about boosters