It wasn't optional. I was Non-Commissioned Officer in Charge of Public Health at a large Air Force Base, and I oversaw the admin parts of the Immunization Clinic. I reported number/percentages of deployees vaccinated, I kept track of adverse reactions and refusals. A person who refused the vaccine could not be deployed to areas for which it was required. If they couldn't be deployed, they couldn't do their jobs, and their fellow airmen, and those in their chain of command couldn't trust them to be there in the event of an attack.

There were a few who were discharged.