An ‘adverse event following immunisation’ (AEFI) is any untoward health event which happens after a person receives a vaccine. 

A ‘health event’ is a symptom (something with a person complains of, for example a ‘headache’ or ‘difficulty seeing’) or a ‘sign’ (something a health practitioner notices about a patient, for example, raised blood pressure). The health event may or may not be caused by the vaccine.

For example, the following are adverse events following immunisation: a person who receives a COVID-19 vaccine and then has a stroke, a heart attack or a death in a motor vehicle accident. Any of these events may or may not be associated with vaccination, but all of these events are ‘AEFI’. An AEFI usually occurs within 28 days following vaccination, but there is no time limit to reporting an event.

All health events after vaccination are important to investigate, because vaccines are given to healthy people.