Depending on severity, an adverse reaction may also be considered a serious adverse event. They often resolve on their own, and may or may not require medical intervention. The intensity of these reactions may range from mild to moderate to severe. Usually, vaccine side effects are identified during clinical trials. Source: Chapter 2 General Recommendations on Immunization.Īdverse reactions, also known as side effects, are considered to be caused by a vaccine. These reactions are usually mild and can last for several days. Systemic reactions include fever, headache, body aches, fatigue, etc.Injections site pain, redness or swelling are most commonly reported local adverse reactions. They are usually mild and occur within a few hours of the shot. Local reactions occur where the shot was given.