An adverse event following immunization (AEFI) is an unwanted or unexpected health effect that occurs after an immunization is given. AEFI may include local reactions at the injection site, allergic reactions, and other conditions or reactions. It is important to note that an adverse event that follows an immunization may, or may not, be due to that immunization.
Ontario public health units are required to follow up potential AEFI that are reported by health care providers. Reported AEFI that meet the provincial case definition are included in the provincial, national, and international system of vaccine safety monitoring.