Vaccine side effects like soreness, fever, and fatigue scare some people out of getting their shots. Ask a doctor, though, and they’ll tell you these unpleasant symptoms have a silver lining: they’re a signal that your immune system is firing up in response to the shot, doing exactly what it’s supposed to do. [time-brightcove not-tgx=”true”] “If you feel bad after the vaccine, at least feel good about feeling bad,” says Dr.