investigate health concerns, but the study population is not large enough to examine the rare adverse events associated with vaccines (Miller, 2012). The Hospital Episode Statistics (HES) database contains records for all hospital admissions in the United Kingdom, along with the individual’s NHS number for each admission. Using the NHS number, researchers can contact an individual’s general practitioner to obtain immunization records and link those data to any hospital admission from the HES.
England and Wales maintain national child health databases that routinely collect immunization records and can likewise be linked with the HES by use of an NHS number and specified approvals. This method has been used to investigate adverse event signals, such as a suspected increased risk of purpura or convulsions from the meningococcal group C conjugate vaccine and a potential association between MMR and idiopathic thrombocytopenic purpura (Andrews et al., 2007; Miller et al., 2001).
Denmark is uniquely positioned to build and maintain large cohorts for the evaluation of vaccine safety thanks to the Danish Civil Registration System (CRS) and the national health care system. The CRS was established in 1968 and registered every living person in Denmark at that time. Every living resident in Denmark, including noncitizens, is issued a unique personal identification number, and the CRS collects data on each individual’s gender, date of birth, place of birth, place of residence, citizenship status, and parents and spouses, and the CRS continuously updates vital statistics (Pedersen et al., 2006).
Linking a personal identification number to the data collected by the CRS makes it possible to track demographic trends and vital statistics for Danish residents over time. This identifier is also used to link individuals with data collected by Denmark’s many health care registries. The National Board on Health administers registries on the incidence of specific diseases (e.g., the National Diabetes Register and the Danish Cancer Register), and since 1990, Denmark has maintained a registry containing information on all vaccinations administered to children aged 18 years and younger. General practitioners report incidences of vaccination to a state-based administrative registry and are in turn reimbursed by the national health insurance system (Thygesen et al., 2011).
Epidemiological research on vaccine safety is conducted with data from these registries by the Department of Epidemiology Research at the Statens Serum Institut, one of Denmark’s largest health research institutions (Statens Serum Institut, 2012). Because each health-related registry records the resident’s CRS, it is possible to link the data collected by separate registries. Therefore, much of the formative research on vaccine safety