Immunization is arguably the most effective, lowest cost and safest intervention to improve health in the past century. Occasionally side effects do occur. All possible side effects after immunization, called ‘adverse events’, are recorded. According to the World Health Organization, an adverse event following immunization (AEFI) is any unpleasant medical occurrence following immunization, which may or may not, be related to the administered immunization. In Canada, the Canadian Adverse Events Following Immunization Surveillance System (CAEFISS) and the Immunization Monitoring Program ACTive (IMPACT) monitor adverse events after immunization.
The risks associated with vaccines are much lower than the costs of the diseases they prevent. The most commonly reported symptoms are localized swelling, red arm and a mild fever (less than 38ºC). These reactions can be part of the body’s normal response to the vaccine and do not usually last long. Approximately one person in every one million doses of vaccine distributed experiences a severe allergic reaction called anaphylaxis. This reaction can be effectively treated by the nurse or doctor administering the vaccine. The questions that patients are asked before getting a vaccine are designed to screen out the people at highest risk of severe reactions.
Simcoe Muskoka
Technical Notes
Simcoe Muskoka
Safety and Effectiveness
The figure below shows the percentage of parents of school children (5-17 years old) in Simcoe Muskoka population who strongly or somewhat agreed that immunizing children protects them from disease (95.8) and that vaccines are safe for children (93.9%).
Adverse Events Following Immunization
In Simcoe Muskoka in a typical year, there are between 15 and 70 adverse events following immunization per year among the hundreds of thousands (100,000+) of vaccines administered by the health unit and distributed to health care providers each year (iPHIS, Panorama). Among these cases, the most commonly reported symptoms are localized swelling, red arm, and mild fever (less than 38 degrees Celsius). There have been no deaths (iPHIS).
For more information on vaccine safety, see one of the following credible sources:
Technical Notes
The Ontario Health Protection and Promotion Act requires health care providers to report adverse events following immunization to the health unit.
The safety and effectiveness of vaccines was determined through asking respondents to the RRFFS survey in 2012-13 of their agreement with the statements, “By immunizing children we are protecting them from disease”, and “Vaccines are safe for children to have.
A Note on Cause and Effect:
Just because adverse events occur after immunization does NOT mean they were necessarily caused by the immunization.
In order to determine whether adverse events are caused by immunization, researchers conduct large studies called clinical trials. These studies only occur after the vaccine has been proven safe in multiple stages of testing by the Biologics, Radiopharmaceuticals and Genetic Therapies division of Health Canada. Researchers recruit thousands of participants and divide them into two groups: one group receives the vaccine and the other group receives a placebo (e.g. a shot of salt water). If the adverse events occur equally in both groups, then the vaccine is not the cause because people who did not receive a vaccine are experiencing the same events. Only by comparing the two groups can researchers make any conclusions. Vaccines must be proven safe in large clinical trials before being licensed for public use.