Although they are less demanding in time and cost than a cohort study, the committee concluded that studies with case-control designs are unlikely to advance knowledge and provide answers to the four primary research questions of interest to concerned stakeholders presented in Box 6-1. The main reasons for this conclusion are that (1) the major variations in immunization history of interest are relatively uncommon, necessitating the enrollment of a large number of affected cases and unaffected study participants, and (2) it is not clear how accurately investigators would be able to retrospectively reconstruct details of the child’s vaccination history. In addition, case-control studies can be used only if the adverse event of interest is known (see Appendix D for further discussion). Additional methodological work designed to determine the accuracy of retrospective ascertainment of vaccine histories and known adverse events may well be warranted.
Secondary Analyses of Existing Databases
Unlike prospective observational studies, which require the collection of new data, secondary analyses of accumulated data, such as retrospective cohort or case-control studies, are traditionally less resource intensive because they generally rely largely on information previously or routinely collected in existing databases. Given the comprehensive state of immunization data systems in the United States, the committee considered secondary analyses with data from existing data sets to be the most feasible option for the study of the safety of the childhood immunization schedule. In particular, a number of questions about variations in the current immunization schedule could be further investigated by the use of VSD.
VSD is the premier electronic health record (EHR)-based vaccine safety data system in the United States (Baggs et al., 2011; Chen et al., 1997; DeStefano, 2001). As noted in Chapter 3, VSD is a collaboration between the CDC and nine health plans that serve about 9.5 million members and that have an annual birth cohort of more than 100,000. In recent years, funding for VSD has totaled approximately $9 million per year, with additional funding being provided for special projects, making VSD a relatively low-cost and effective data system for investigating immunization safety (Frank DeStefano, CDC, personal communication, September 25, 2012).
VSD could be valuable for answering the research questions that the committee identified in Box 6-1 because it includes information on the immunization histories of participants that can be used to identify