The carrier said it would start on Tuesday the process of firing 593 employees who decided not to get vaccinated. “This was an incredibly difficult decision but keeping our team safe has always been our first priority,” Chief Executive Scott Kirby and President Brett Hart told employees in a memo.

The workers can save their jobs if they get vaccinated before their formal termination meetings, the company officials said. United has received requests for vaccine exemptions from employees for religious and medical reasons.

Those employees account for less than 3% of the airline’s 67,000 U.S. workforce, United officials said. The company had plans to put employees who received religious exemptions on temporary, unpaid personal leave from Oct. 2.

Those plans, however, have been put on hold until Oct. 15 because of a lawsuit challenging the policy. Excluding those who have sought an exemption, United said more than 99% of U.S.-based employees have been vaccinated against COVID-19.

A company spokesperson said the airline plans to hire about 25,000 people over the next few years, and vaccination will be a condition of employment for all new hires.

United will also require students at its pilot training school to get vaccinated, the spokesperson said. The company dismissed the notion that the vaccine requirement was deterring applicants for jobs at the air carrier.

United received 700 applications for about 400 job postings last month at a Denver career fair. Similarly, it has received more than 20,000 applications for about 2,000 open positions for flight attendants, the spokesperson said.