has been conducted in Denmark with registry linkages. These linkages of data between the childhood vaccination registry and other disease-specific registries provide data that can be used to evaluate hypotheses on vaccine safety for large cohorts of Danish residents (often, more than 500,000). For example, the cohort study design has been used to investigate associations between MMR and autism, childhood vaccinations and type 1 diabetes, and thimerosal-containing vaccines and autism (Hviid et al., 2003, 2004; Madsen et al., 2002).
Canada’s health care system has some similarities with those in countries such as Denmark and the United Kingdom, including the provision of primary care health services without cost sharing. Unlike those countries, Canada’s health care system is provincial, rather than federal, meaning that coverage varies across the 13 separate provinces. The determination of an immunization schedule is no exception: each province is given authority to create its own immunization schedule, although evidence of vaccine safety and efficacy is still reviewed by the National Advisory Committee on Immunization. Nevertheless, provinces may have very similar schedules for one vaccine; for example, the only province that does not recommend immunization with MMR at 12 months of age is Prince Edward Island, which recommends the vaccine’s first administration 3 months later at age 15 months. For another vaccine, that for hepatitis B, the differences are more striking: the province of Prince Edward Island recommends administration of the first dose in infancy, whereas its provincial neighbor, Nova Scotia, does not recommend administration of the first dose until grade 8 (Macdonald and Bortolussi, 2011).
Canada also has a spontaneous reporting system for suspected adverse events related to vaccines, the Vaccine Associated Adverse Event Reporting System, which was established in 1987. Today, the passive surveillance system is called the Canadian Adverse Events Following Immunization Surveillance System and is maintained by the Public Health Agency of Canada. Health care professionals in Canada can submit reports of suspected adverse events to their local public health authority. Unlike in the United States, however, Canada has no system for the general public to report events without a health professional, who must submit the required form. In the provinces of Manitoba, New Brunswick, Nova Scotia, Ontario, Quebec, and Saskatchewan, reporting of adverse events after immunization is required by law (Public Health Agency of Canada, 2006).
To supplement its passive surveillance system, Canada implemented the Immunization Monitoring Program, Active (IMPACT) in 1991. The IMPACT network is based in 12 pediatric hospitals and is maintained by