United Airlines said Tuesday that 593 of its employees are facing termination for failing to comply with its Covid-19 vaccination policy, one of the strictest mandates for inoculation from a U.S. company.

Over 96% of United’s 67,000 U.S. employees met the vaccination requirement. Late Monday was the deadline for proof of vaccination or to receive the first dose of a 2-dose vaccine.

The mandate was announced by the airline this summer and granted exemptions to approximately 2,000 United workers. The Chicago-based airline had said staff to whom it grants such exemptions will be placed on temporary unpaid leave.

And we are aware that this was not an easy decision for many. But there’s no doubt in our minds that some of you will have avoided a future hospital stay — or even death — because you got vaccinated,” United’s CEO, Scott Kirby, and the company’s president, Brett Hart, told employees in a note Tuesday.

Employees who are not vaccinated and do not have exemptions from the requirement could be fired, although this process can take several weeks. United’s Kirby Hart and United said that while this was an extremely difficult decision, keeping our team safe is always our top priority. The staff that failed to upload proof of vaccine included mechanics, pilots, flight attendants, or flight attendants. A spokesperson declined further details.

A United spokesperson said that the company was willing to help unvaccinated workers during termination if they decide not to get inoculated. According to the spokesperson, terminations are not expected to cause operational issues for United.

Employers who are fired because they have not been vaccinated will be dismissed on grounds that the company’s safety policy was violated. This could render them unemployed.

CNBC earlier Tuesday reported that dozens of employees shared their vaccination cards with the company within the last days.

CNBC reported that the Association of Flight Attendants stated that the flight attendants who didn’t submit their vaccination cards, or had not received exemptions, fell about half over the weekend and then fell to 100 by Tuesday. This is according to roughly 23,000.

The International Association of Machinists and Aerospace Workers represented more than 500 United workers. However, that number dropped to less than 400 by Tuesday afternoon according to Mike Klemm, District 141 president. More than 25,000 United citizens are represented by the union. He said that 700 more had been granted exemptions. This group also includes passengers service and fleet workers.

Klemm stated that the union plans to file grievances for wrongful termination if workers refuse to get vaccinated.

In federal court, six United Airlines employees filed a lawsuit against the carrier. They claimed that the company did not provide “reasonable accommodation” to accommodate their religious and medical needs. United stated that it would “continue vigorously defend” its policy.

U.S. companies have increasingly issued vaccine mandates for some or all of their employees, from Tyson Foods to Walmart and McDonald’s since Covid cases spiked over the summer.

President Joe Biden earlier this month said his administration plans to mandate that large companies require their employees be vaccinated or have them test regularly for Covid. Executives at airlines say that they await the details.

Although all major U.S. airlines have encouraged employees to get immunized, some have used different approaches. Some have offered extra time or pay as incentives. Many have not required vaccinations.

Delta Air Lines Plans to add $200 per month to the company’s health-care bills for unvaccinated workers in November. Delta, along with Alaska Airlines and American Airlines have said unvaccinated employees will have to use their own sick time if they miss work because of Covid. Hawaiian Airlines Staff must have their vaccines up-to-date by November 1.

Even though an airline does not need vaccines it can affect the flight schedules of some employees. According to a memo from staff, American Airlines informed pilots that Canada and Suriname will require them to get vaccinated in order to fly those flights. According to the union, this also applies to flight attendants.

American anticipates that more countries will join the list.

United requires pilots and attendants who fly to certain locations to have their vaccinations since August 1. It includes, among other places, Brazil, Peru and India.

American Airlines and Southwest Airlines pilots’ unions said that pilots shouldn’t be required to have vaccines. Last week, American’s mainline pilots were represented by the Allied Pilots Association. They asked the White House and Transportation Department to offer pilots an alternative to a mandatory federal vaccine. According to union, approximately 4,200 out of the 14,000 pilots in this group are not vaccinated. According to the APA, a federal mandate for vaccines could cause flight disruptions and holiday labor shortages.