Ensuring Safe immunization

Karina Top describes the ways that data about vaccines are collected and used in Canada to ensure that immunization programs are safe.


Immunization is considered one of the 10 great public health achievements of the 20th century. Immunization programs have saved millions of lives and reduced the rates of diseases such as measles, diphtheria, and polio by more than 99%.  However, as rates of vaccine-preventable infections have declined, our collective memory of these serious diseases has faded. As a result, the public’s focus has shifted to the safety of recommended vaccines.

Vaccines are considered very safe, with benefits outweighing risks. However, adverse events are occasionally observed, most frequently fever, and swelling, pain, or redness at the injection-site. In rare cases (approximately 1–2 per 10,000 vaccinated individuals), adverse events can be severe enough to come to medical attention, for example allergic reactions or seizures. The continued success of immunization programs depends upon maintaining public confidence in the safety of recommended vaccines through rigorous surveillance and research.

vaccineVaccine safety is tested at all stages of vaccine development: first in animal studies, second in human clinical trials, and third, after the vaccine is publicly available as part of a public health program. The ongoing safety of recommended vaccines is monitored by the Canadian Adverse Event Following Immunization Surveillance System, a collaboration of the Public Health Agency of Canada and the federal, provincial, and territorial health jurisdictions. Public health authorities conduct passive surveillance for severe or unexpected adverse events following immunization based on reports from healthcare providers. As well, the Canadian Immunization Monitoring Program, ACTive, searches for select adverse events among children admitted to any of the 12 participating pediatric hospitals. The data collected by the public health authorities and the Canadian Immunization Monitoring Program ACTive is compiled by the Canadian Adverse Event Following Immunization Surveillance System, which continuously monitors for changes in the safety profile of vaccines.

An increase in reports of a particular adverse event following immunization stimulates further investigation and research to determine if the event could be related to a specific vaccine. A vaccine safety concern arising in another country (e.g., seizures with fever after influenza immunization) might also prompt a review of data collected by the Canadian Adverse Event Following Immunization Surveillance System or provincial authorities to determine if similar events have been reported with the same vaccine in Canada. When an adverse event is associated with a specific vaccine, there will be a risk assessment. As a result of this assessment, immunization recommendations may be modified, or the vaccine may be withdrawn from the market.

Adverse events following immunization may also raise concerns at the individual level. When an individual develops a serious medical event after vaccination there may be questions regarding the role of the vaccine and the risk of another adverse event with future immunizations. The Canadian  Special Immunization Clinics Network is a group of infectious diseases physicians and allergists across Canada with expertise in the evaluation and management of patients with adverse events following immunization. Special Immunization Clinic physicians conduct standardized patient assessments in order to determine whether the vaccine was likely to have caused the adverse event and to estimate the risk of a similar adverse event after the next immunization. The physicians make individualized recommendations regarding future immunizations and follow patients after re-immunization. Patient data are collected in a national database to determine the risk of recurrence of adverse events upon re-immunization in order to guide immunization recommendations for these high-risk patients.

Healthcare providers and the public can be reassured that the benefits of vaccines far outweigh the risks. Public health authorities and researchers are actively engaged in monitoring and evaluating immunization programs and expert physicians are available to provide care to patients who experience adverse events. Together, these groups are working to ensure that immunization programs remain as safe as possible.


Karina Top is pediatric infectious diseases specialist and an Assistant Professor of Pediatrics and Community Health & Epidemiology at Dalhousie University.  @drkarinatop


Conflict of Interest: Karina Top receives research funding from the Public Health Agency of Canada and Canadian Institutes of Health Research as a Principal Investigator in the Canadian Immunization Research Network, and from the World Health Organization. She also receives research funding and salary support from the IWK Health Centre.