The U.S. Air Force on Monday said 27 service members had been discharged for refusing to receive a COVID-19 vaccine, the first active-duty troops believed to have been removed for declining the vaccine.

The Pentagon made the vaccine mandatory for all service members in August and the vast majority of active-duty troops have received at least one dose.

Ann Stefanek, a spokeswoman for the Air Force, said the troops were given a chance to explain why they had refused to get vaccinated, but none of them were given exemptions.

About 97% of Air Force personnel are vaccinated against the virus, a number far higher than the general U.S. population. The Air and Space force has about 326,000 active duty personnel.