system to address the primary research questions of interest and increase the generalizability of research results.

Further discussion would be required to assess the feasibility and cost of such efforts. The committee noted that although VSD represents the most promising system for investigating outcomes after immunization with the recommended childhood immunization schedule, other resources discussed in Chapter 3, such as VAERS, the National Immunization Survey, and immunization information systems, are highly valued resources for monitoring vaccine safety and coverage as well. The Post-Licensure Rapid Immunization Safety Monitoring (PRISM) program, which has been used to evaluate vaccine safety in a larger cohort than the VSD, may have the capability to monitor rare adverse events potentially associated with the childhood immunization schedule. However, the data are not yet well-characterized.

Analyses of comparable international immunization surveillance systems in countries including Denmark, the United Kingdom, and Canada have historically been better suited for these purposes for the reasons described below. Although consideration of international immunization surveillance systems was not central to the committee’s task, analyses in Denmark, the United Kingdom, Canada, and other countries also hold considerable promise for advancing knowledge about the health outcomes associated with the immunization schedule. First, as discussed in Chapter 3, these countries often collect and maintain full immunization histories for the entire population, greatly increasing the total sample size and the number of children immunized with less common combinations of vaccines (including no vaccines). Second, many of these countries have comprehensive health and educational registries permitting linkage to longer-term and less severe child outcomes. Third, these systems include a richer set of variables on sociodemographic characteristics and family history, permitting analyses of potentially susceptible subpopulations.

The committee considered but does not recommend cross-national comparisons because of the potential bias and lack of generalizability from results that must account for different environments, vaccine antigens, or immunization schedules. The U.S. population differs from the populations in other countries in important ways, including on the basis of genetics and health care history. Even vaccine efficacy can vary among populations, as has been demonstrated in separate studies of a Haemophilus influenzae type b conjugate vaccine in two different populations (Eskola et al., 1990; Ward et al., 1990). A cross-national comparison to study child health outcomes related to recommended childhood immunization schedules would require careful and extensive consideration of the possible covariates, many of which may not be known at this time. Ecological comparisons may be useful for monitoring disease trends and detecting epidemiological signals; however, the information gathered from such studies could not be

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