this report, a small percentage of the U.S. population receives no recommended childhood immunizations for reasons ranging from religious or philosophical beliefs, such as followers of Christian Science and some in U.S. Amish communities, to health reasons, such as children with certain conditions, to personal convictions about the safety of vaccines. Given the above-average proportion of unimmunized children in these populations, ranging from 4 to 16 percent in surveys of different communities (Smith et al., 2004; Wenger et al., 2011), it has been suggested that such a population could serve as a naturally occurring unimmunized group in designing a new prospective cohort study. However, such a study would have limited utility to accurately assess differences in health outcomes between unimmunized and fully immunized children. First, there are questions regarding the potential size and resulting statistical power for such a study. As with RCTs, sufficiently large numbers of participants would need to be recruited for each study arm—those who are unimmunized and those who are fully immunized. Because some Amish communities and other potential naturally occurring unimmunized populations have relatively so few unvaccinated children, the sample population of unimmunized children who could be recruited would likely be too small to provide adequate statistical power, particularly for very rare outcomes (see Appendix D).

Furthermore, the study would need to account for the many confounding variables that distinguish distinct subgroups of naturally occurring unimmunized populations from the rest of the U.S. population, including lifestyle factors and known genetic variables that may play a role in the development of allergies, asthma, and other conditions. For example, data from the National Immunization Survey have shown that unimmunized children are characteristically different from children who are underimmunized or fully immunized on the basis of race, gender, socioeconomic status, and parental concerns (Smith et al., 2004). For all these reasons, the committee does not recommend the pursuit of prospective cohort studies with distinct subgroups of naturally occurring unimmunized populations (such as those who decline immunizations due to membership in specific religion or cultural groups).

One option warranting additional investigation would involve embedding a new prospective cohort study of nonvaccinated and fully vaccinated families within the VSD surveillance system. If adequate numbers of fully unvaccinated children were included within VSD, it might be possible to identify comparable, well-matched, fully vaccinated children and actively monitor both groups over time with direct assessments of health functioning. In contrast to a study of, for instance, Amish families only, this study would likely include a more diverse and less highly-selective group of unimmunized children (with reduced potential for confounding) and with a larger sample size.

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