Is it safe for a person to get a flu shot while on a course of antibiotics or should they wait until after they've finished their prescription? if wait, how long?

Should not matter: It should not matter whether a person is using antibiotics or not. Such a person may get the annual flu shot, if there is no other reason why she should not get it. Antibiotic usage does not change one's decision about a flu shot. If the person is feeling poorly due to an underlying condition that created the need for antibiotics, the person usually can wait a week until he or she feels better.