In August, United became the first major U.S. airline to issue a vaccine mandate for its employees. Employees were given until five weeks after the Pfizer vaccine was granted full approval or until Oct. 25 to present proof of being fully vaccinated. As the Food and Drug Administration granted the Pfizer vaccine full approval on Aug. 23, employees will have until Sept. 27 to get fully vaccinated.

"We have no greater responsibility to you and your colleagues than to ensure your safety when you're at work, and the facts are crystal clear: everyone is safer when everyone is vaccinated," United CEO Scott Kirby and President Brett Hart said to employees in a memo obtained by The Hill last month.