Further investigation of the number and characteristics of fully unvaccinated children or children vaccinated by use of alternative schedules within VSD appears warranted. It would be important to ensure an adequately comparable comparison group of fully vaccinated children. The committee raised some concerns that differences between the comparison groups of interest might constrain the utility of such a study, for reasons discussed below in regards to secondary analyses.

Furthermore, to be of sufficient scientific quality, such a study would require considerable effort to retain study participants. Additional consideration should be given to the feasibility of assessing long-term health outcomes for participants in VSD and the cost of doing so. This information would be essential to adequately assess the feasibility and cost of initiating a new prospective cohort study nested within VSD.

In addition to studies focused on existing unimmunized populations, the committee recognized that other longitudinal cohort studies of infants and children could be informative for evaluating long-term health outcomes after immunization, if a large sample size was available and accurate recording of immunization coverage was possible. One such opportunity is the National Children’s Study (NCS), which is funded by both the U.S. Congress and the National Institutes of Health through the Children’s Health Act of 2000 and which received total funding of $744.6 million from fiscal years 2007 to 2011. The budgetary request for fiscal year 2013 is $165 million, which will fund the continuation of the pilot study and introduction of data collection for the main study (National Children’s Study, 2012).

The main NCS will be a multicenter effort that will examine the effect of a child’s environment—including variables such as air and water quality, diet, family dynamics, and cultural influences—on his or her general health and well-being from birth through age 21 years. With a target population of 100,000 children, the NCS will be adequately powered to evaluate rare health outcomes and will aim to prioritize the investigation of environmental determinants of neurodevelopmental disorders and asthma, among other outcomes. Once begun, the main study will actively collect immunization histories. NCS therefore affords an opportunity to study potential health outcomes among children with a range of immunization histories, and the committee encourages such efforts through NCS and other similar cohorts to create a rich set of data for continued research.

Given the opportunity available through NCS, the limits of studying distinct subgroups of naturally occurring unimmunized populations, and the high cost of pursuing prospective data collection, the committee does not consider the initiation of new prospective cohort studies to be the most feasible or fruitful approach to studying the recommended immunization schedule at this time.

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