Federal contractors will have broad leeway to enforce President Joe Biden’s Covid-19 vaccine mandate, according to new guidance the White House released on Monday, laying out details on implementation of the rules.

Under the new guidance, federal contractors from IBM and Boeing to food service providers will have flexibility to determine how they enforce the vaccination requirements for workers who refuse to be vaccinated.

“A covered contractor should determine the appropriate means of enforcement with respect to its employee at a covered contractor workplace who refuses to be vaccinated and has not been provided, or does not have a pending request for, an accommodation,” said the guidelines, which affect millions of workers.

The federal contractor guidelines are stricter than the forthcoming vaccine mandate for businesses with 100 or more employees, which allow for regular testing broadly as an alternative to the vaccine. The Labor Department is still finalizing those rules. Businesses have asked for that mandate to be delayed until after the holiday season over concerns about possible supply chain disruptions.

The White House released the federal contractor guidance Monday after contractors sought more details on how to implement the rules. Biden issued an executive order on Sept. 9 requiring federal contractors to ensure their employees are vaccinated against Covid-19 and follow masking and social distancing policies. The administration set a Dec. 8 deadline for contractors to implement those requirements.

Senior administration officials made clear that Dec. 8 is not a hard deadline for contractors to have all of their employees fully vaccinated. Instead, contractors must demonstrate they are making a good faith effort to ensure employees are getting vaccinated and have plans in place to ensure masking and social distancing policies are followed in the workplace.

The federal government will defer to contractors to determine when an employee has a sincerely held religious belief or medical condition that requires accommodation, according to senior administration officials. Federal contractors are not required to make a final determination on accommodation requests when an employee begins work.

“The covered contractor may still be reviewing requests for accommodation as of the time that covered contractor employees begin work on a covered contract or at a covered workplace,” the guidelines said.

However, federal contractors must require employees with pending accommodation requests to abide by policies on masking and social distancing while their requests are under review, according to the guidelines.

Federal contractors including some large airlines such as Southwest and American, and aerospace giant Boeing have said employees must be vaccinated by the Dec. 8 deadline or apply for an exemption.

Some labor groups have opposed the mandate, including pilots’ unions at American and Southwest. The latter sought to bar the implementation of the mandate, a request a federal judge in Texas denied last week.

American and Southwest executives have softened their tone over the mandate, urging employees to apply for religious or medical exemptions if they don’t plan to get the vaccine, and said they don’t expect to terminate employees over it. Southwest last month dropped a plan to put workers with pending exemption requests on unpaid leave. Airline executives said they don’t foresee the mandate impacting flights over the holidays.

Federal agencies could bar a contractor employee who refuses to be vaccinated from entering a federal workplace, according to the guidelines.

“In most circumstances individuals who are not fully vaccinated need to follow applicable masking, physical distancing, and testing protocols,” the guidelines said.

Federal contractors won’t have to show proof of vaccination rates at the deadline, a senior administration official said. But noncompliance could result in the loss of a federal contract.

Eleven Republican-led states sued the administration on Friday, arguing that the vaccine mandate is unconstitutional. The administration has made clear that the requirements supersede any state laws that bar compliance with Covid-19 mitigation policies.