Unlike other airlines which are federal contractors and subject to stricter rules when it comes to their employees’ decisions to get vaccinated against COVID-19 or not, Spirit Airlines will give employees an option to get regular COVID-19 tests—though employees will need to agree to one of the two policies.

In an email Friday, CEO Ted christie told staff that if they wanted to avoid the upcoming federal vaccine mandates for larger private companies, that they would need to get their vaccines soon, though testing would be an option when the mandate went into effect.

“The federal government has made it clear that COVID-19 vaccination or testing requirements will be mandated for companies with more than 100 employees, and we will comply with that order,” Christie said in the email, which was obtained by CNBC.

The company, which had 8,700 employees at the end of 2020, does not have to follow the same rules as some of its rival airlines, such as American Airlines, JetBlue, Alaska Airlines and Southwest, all of which are federal contractors and held to a stricter standard that doesn’t give employees a testing option. Employees of those airlines are expected to comply with the rules by Dec. 8.

The rules regarding airlines and employee vaccination status have seen a change in how some of the airlines, like Alaska Air, approached their methods to get employees vaccinated. While they initially gave those who had proof of vaccination a $200 bonus for getting the shot, now the employees are all required to be vaccinated unless they have a valid medical or religious exemption, or they won’t be able to work.

United Airlines required employees to be vaccinated by Sept. 27, and became the subject of a class-action lawsuit from employees who said that the company failed to authorize any exemptions and was not offering adequate severance pay for those who opted not to get the shot.

Spirit Airlines A Spirit Airlines plane is parked on the tarmac as another Spirit Airlines passenger aircraft taxis after landing at Dallas/Fort Worth International Airport, Dec. 12, 2018. Photo: Photo by Robert Alexander/Getty Images