Finally, it might be conceivable to conduct direct assessments of subgroups of interest (e.g., those who receive no vaccinations and a comparable group that receives the full immunization schedule). This option is discussed further below, but it is more feasible to study children who have had incomplete immunizations by a specified age than to identify children considered vaccine refusals because the population which falls into the latter category is generally very small.

Extending the Length of Follow-Up of VSD Patients

A limitation of VSD is that it includes data only from individuals in the nine participating health plans. Families with young children may move and switch health plans, resulting in limited follow-up information after their immunizations. This shortcoming is largely overcome in comparable systems in Scandinavia and the United Kingdom because of their universal health care systems and patient registries that contain information on medical services received from primary care providers. The use of strategies to collect health care utilization data through EHRs or provider reports after a participant has left the original health plan may warrant consideration.

Increasing the Number and Variety of VSD Participants

With an annual birth cohort of more than 100,000 participants, the total number of children monitored through VSD is substantial. However, national estimates derived from a representative sample of all U.S. children, including those in public health plans, suggest that less than 1 percent of children receive no vaccines. Data from VSD (Jason Glanz, University of Colorado–Denver, personal communication) suggest that the number of unvaccinated children within VSD is generally consistent with national values. Approximately 1.23 percent of children participating in VSD had no vaccinations recorded by age 1 year, and 1 percent of children had no vaccinations recorded by age 2 years. These estimates are limited to children who were born between 2004 and 2008 and who had a minimum period of enrollment in VSD of 12 months and a maximum enrollment of 36 months. It is not clear how commonly other variations of the recommended immunization schedule occur among the children in VSD.

In addition, the diversity of the participants represented in VSD is limited by the fact that managed care organizations in the Southwest and rural South are not currently among the managed care organizations participating in VSD. Furthermore, because VSD does not now include any public insurance plans, its population has fewer low-income and minority individuals than the number in the U.S. population as a whole. Options to broaden the diversity of VSD participants would enhance the utility of this



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