The union representing United Airlines pilots is expressing concern over the carrier’s announcement it will mandate a COVID-19 vaccine for all U.S. employees – under threat of termination.

“Employer-mandated vaccinations are an issue that must be bargained,” the Air Line Pilots Association said in a statement to Barron’s.

United CEO Scott Kirby and President Brett Hart said Friday the airline would require each of the company’s 67,000 U.S. workers to be fully vaccinated by Oct. 25 or face possible loss of employment.

United will health and religious exemptions on a case-by-case basis but if it grants exemptions those employees will have to wear face masks at all times.

The Pilots Association, the Association of Flight Attendants, and the Teamsters all issued statements suggesting their respective unions will accept the mandate and, in fact the pilots union said more than 90 percent of its members are already vaccinated, according to CNN.

Nonetheless, the mandate “warrants further negotiations to ensure our safety, welfare, and bargaining rights are maintained,” the pilots union said, but also added that an “employer mandate would be determined to be lawful” if it were to be challenged in court.

The union representing the flight attendants said in a statement that “Experts agree, vaccination is our best defense against Covid-19 and the extension of harm. We’ve seen a notable uptick of test positives over the last two weeks, [the] majority of which are unvaccinated Flight Attendants, though some breakthrough infections have been recorded.”