immunization schedule that compare safety outcomes in fully vaccinated children with those in unvaccinated children or those vaccinated by use of an alternative schedule.

The committee also reviewed opportunities to study groups that choose not to vaccinate their children by use of a prospective cohort study design. However, such a study would not conclusively reveal differences in health outcomes between unimmunized and fully immunized children for two main reasons. First, the sample populations often suggested for study (such as some religious populations) may be too small to adequately power such a comparative analysis, particularly for very rare adverse health outcomes. Such a study would also need to account for the many confounding variables that separate these naturally occurring unimmunized populations from the average U.S. child, including lifestyle factors and genetic variables.

The committee finds that secondary analyses of existing systems are more promising approaches to examination of the research questions that the committee identified in future studies of the childhood immunization schedule. The Vaccine Safety Datalink (VSD) is a useful collaborative project that could conduct both postmarketing surveillance and longer-term targeted research. The ability to augment routinely collected administrative data in VSD with data from parent interviews and reviews of medical records for a selected study population is an important strength.

VSD is currently the best available system for studying the safety of the immunization schedule in the United States. VSD should strive to improve the generalizability of its data to the U.S. population as a whole by enhancing the quality of its demographic information and by expanding its scope to include more diversity in its study populations. Secondary analyses with data from other existing databases (that might be modeled on VSD) could be a feasible, ethical, and cost-effective means of investigating several research questions that the committee identified. The committee recognizes that the commitment to VSD studies by the managed care organizations currently receiving funding through VSD needs to be sustained to continue to build on existing efforts. The committee concludes that VSD is a valuable component of the federal research infrastructure and will be the best-suited source of data for studying the childhood immunization schedule. Its utility will be expanded with the addition of more detailed demographic data and family medical histories.

Recommendation 6-3: The committee recommends that the Department of Health and Human Services (HHS) and its partners continue to fund and support the Vaccine Safety Datalink project to study the safety of the recommended immunization schedule. Furthermore, HHS should

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