Adverse Events Following Immunization (AEFI)


Canada’s long-standing vaccine safety system ensures that vaccines marketed and used in Canada are as safe as possible. Vaccines, like all medicines, must go through a series of steps before they are approved for use. Health authorities in Canada take vaccine safety very seriously.

Before any vaccine is approved for use in Canada, it must be shown to be safe and effective in preventing the disease that it targets through multiple clinical studies. Most common side effects (also called adverse events) of a vaccine are identified in studies before the vaccine is licensed, but rare adverse events may not be detected in these studies. Therefore, the Canadian vaccine safety system continuously monitors for possible adverse events after a vaccine is licensed. When millions of people receive a vaccine, less common adverse events that were not identified earlier may occur.

If a link is found between a possible adverse event and a vaccine, public health officials take appropriate actions that include determining if the recommendation for using the vaccine should change.

Monitoring Vaccine Safety

Vaccines are held to the highest standards of safety. Canada has one of the safest, most effective vaccine supplies in history. Vaccines undergo a series of testing before they can be licensed. Once in use, vaccines are continually monitored for safety and efficacy.

Vaccines, like any medication, can cause adverse events. However, a decision not to immunize a child also involves risk. It is a decision to put the child and others who come into contact with him or her at risk of contracting a disease that could be dangerous or deadly.

Video © 2016 Public Health Agency of Canada

How are Adverse Events Following Immunization monitored?

An adverse event following immunization (AEFI) is any untoward medical occurrence in a vaccinee which follows immunization and which does not necessarily have a causal relationship with the administration of the vaccine. The adverse event may be any unfavourable and/ or unintended sign, abnormal laboratory finding, symptom or disease. The Canadian Adverse Events Following Immunization Surveillance System (CAEFISS) monitors adverse events following immunization from across the country through local public health units. The surveillance system seeks to ensure the continued safety of vaccines on the Canadian market.

Vaccine side effects

The most common side effects are mild. On the other hand, many vaccine-preventable disease symptoms can be serious, or even deadly. Even though many of these diseases are rare in this country, they still occur around the world and can be brought into Canada, putting unvaccinated children at risk.

The side effects associated with getting vaccines are usually mild (such as redness and swelling where the shot was given) and go away within a few days. If your child experiences a reaction at the injection site, you can use a cool, wet cloth to reduce redness, soreness, and swelling.

Serious side effects following vaccination, such as severe allergic reaction, are very rare and doctors and clinic staff are trained to deal with them. Observe your child for a few days after vaccination. If you see something that concerns you, call your child’s doctor.

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