Alaska Airlines told its 22,000 employees that the carrier’s work as a government contractor means they must be vaccinated against Covid-19 as early as Dec. 8 because of new federal rules.

The Seattle-based airline hasn’t mandated that staff be vaccinated but has encouraged employees to get inoculated. It has offered extra pay to those who share proof of vaccination with the company.

“Since our company does significant work for the federal government, we have determined that Alaska Airlines, Horizon Air and McGee employees – all part of Alaska Air Group – do fall under this federal vaccine mandate, along with other major U.S. airlines,” Alaska said Thursday in a staff note. ”This means all of our employees, including certain contractors and vendors, will be required to be fully vaccinated, or be approved for a reasonable accommodation such as medical conditions or religious beliefs that prevent them from being vaccinated.”

CNBC saw a copy of the note. A spokeswoman for the airline told CNBC a “significant majority” of the airline’s employees are vaccinated, but she declined to give a percentage, noting that staff are still uploading their proof of vaccination.

Alaska extended its $200 incentive for staff to upload proof of full vaccination from Oct. 15 to Dec. 1.