The Chicago-based airline started the process of firing 593 workers as of Tuesday, though final separation takes weeks. More employees this week uploaded their cards, and by Thursday, just 320 employees hadn’t done so.

United has the strictest vaccine mandate policy of any U.S. airline. In August, the company told its 67,000 U.S. employees that they must be vaccinated against Covid-19 this fall to continue working there. Some 2,000 United employees have sought exemptions for medical or religious reasons.

“Our vaccine policy continues to prove requirements work — in less than 48 hours, the number of unvaccinated employees who began the process of being separated from the company has been cut almost in half, dropping from 593 to 320,” United said in a statement.