As discussed in Chapter 5, the committee noted that limited published data do not provide evidence that the recommended immunization schedule is associated with safety or health risks. Indeed, the available epidemiological data repeatedly indicate the health benefits associated with the recommended schedule (e.g., reduced infections and hospitalizations).

To undertake new studies on the immunization schedule beyond analyses with existing data from surveillance systems, researchers will need to carefully consider the current evidence, both epidemiological and biological, that supports the plausibility of their hypotheses. The decision to initiate further studies should depend on the results of an evaluation of three considerations that the committee identified through its review of stakeholder concerns and scientific findings:

  1. epidemiological evidence of potential adverse health outcomes associated with elements of the immunization schedule (such as postmarketing signals or indications of an elevated risk from observational or experimental studies);
  2. biological plausibility supporting hypotheses linking specific aspects of the immunization schedule with particular adverse health outcomes; and
  3. expressed concerns from some stakeholders about the immunization schedule’s safety, which should support efforts to evaluate the previous two considerations.

Currently, the U.S. Department of Health and Human Services (HHS) considers these criteria before initiating new studies through the Vaccine Safety Datalink (VSD). As discussed in Chapter 3, the Vaccine Adverse Event Reporting System (VAERS) allows parents and providers to report suspected adverse events after immunization. If an association is suspected on the basis of these signals, medical experts in the Clinical Immunization Safety Assessment (CISA) Network evaluate the pathophysiological basis of the suspected event. Researchers may also conduct, using VSD, population-based epidemiological studies on the basis of signals reported through VAERS and conclusions about biological plausibility reported by the CISA Network.

The committee concluded that stakeholder concerns have a role in guiding the research priorities of the Centers for Disease Control and Prevention (CDC), FDA, the National Institutes of Health, and the National Vaccine Program Office because they may point to potential research questions that need to be validated from their epidemiological signals and the plausibility

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