extrapolated to inferences of individual risk of adverse events related to each immunization schedule, and thus would not be useful for shaping U.S. immunization policy.

The major limitations of U.S. surveillance systems to address the primary research questions identified in this report are (1) the potentially limited number of families included in these systems who will have used the major alternative immunization schedules of interest; (2) potentially high rates of migration from the participating health care organizations, resulting in varying and often short-term follow-up after vaccination; (3) limits on how much information on less severe health outcomes is collected from participating children; and (4) limited ancillary information routinely collected about participating children, such as premature birth or a family history of allergies.

Despite these limitations, VSD is currently the best available system for the study of the safety of the immunization schedule in the United States and holds tremendous promise for advancement, including the potential for future prospective cohort studies. Furthermore, continuing to move toward the increased use of EHRs (as encouraged by federal funding), which are what allow VSD to capture and link large amounts of immunization and health data on children, will help the United States establish richer data sets that are more comparable to those in other high-income countries.

To further enhance the data collected by VSD, the system should strive to obtain complete demographic information to strengthen its functions and generalizability to the whole U.S. population. Secondary analyses with data from other existing databases similar to VSD would be feasible, ethical, and a lower-cost approach to investigating the research questions that the committee identified, including research on alternative immunization schedules. To date, the data obtained from VSD have already been used to study health outcomes of children with incomplete immunizations or who may follow alternative schedules, as described above. In addition, the VSD system has a large enough proportion of unvaccinated children to investigate differences in health outcomes of unvaccinated and vaccinated children. Increased efforts to collect information on individual medical histories could lead to a fruitful source of data for studying which populations are potentially susceptible to vaccine adverse events. The committee recognizes that the currently funded managed care organizations’ commitment to VSD studies needs to remain high to continue and build upon existing efforts. Additionally, VSD’s 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

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