Adverse Event Following Immunisation (AEFI)

Otherwise referred to simply as an ‘adverse event’, this is an unexpected medical problem that could happen in volunteers in trials of a candidate vaccine or after the rollout of a vaccine. 

An ‘adverse event’ refers to a medical problem with a vaccine, while ‘side effect’ refers to a problem with a drug, other treatment, or therapy.

Adverse events in clinical trials are generally mild to moderate. Most often, they are caused by factors not related to a vaccine. Serious adverse events help to weed out unsuccessful candidates in pre-clinical trials before they are tested on humans. All phases of clinical trials monitor for any such events to ensure the candidate vaccines or drug is safe enough to proceed to the next phase. 

Adverse events can result in researchers pausing clinical trials until their cause is understood clearly. For example, large Phase lll trials of the Astra Zeneca vaccine candidate for COVID19 were put on hold, when a volunteer developed inflammation of the spinal cord called transverse myelitis. The investigation found it was a rare underlying condition that was not linked to the vaccine candidate. The trial has since resumed.

TIP: Don’t be too quick to link a vaccine to an adverse event. Find out if there is an expert investigation into the matter and wait for the result. Most often the problem is not linked to the vaccine.



Treatment Vaccines


Clinical TrialsCOVID-19Vaccines