concentration of underimmunized individuals within specific regions increases the potential for the transmission of disease.
Children who receive care from more than one health provider may receive too many immunizations. One CDC study reports that 14 percent of young children (aged 19–35 months) were immunized beyond need for the vaccine to prevent polio (Feikema et al., 2000). Children seen only in public health departments were significantly less likely to be extra-immunized. Additional analysis, however, suggests that rates of extra-immunization may be overestimated as a result of documentation errors in medical records (Davis, 2000).
Virtually any and all of the data collection tools discussed below for determining immunization coverage at the national, state, and local levels can be employed or adapted to examine special populations or geographic pockets of need. In addition to these direct measures of immunization coverage, geographic pockets of need can be identified by surrogate measures, including demographic and socioeconomic variables such as average income level, percent of the population receiving Medicaid, and maternal education, all of which are associated with underimmunization (Santoli et al., forthcoming). Surrogate measures have the advantage of being readily available and inexpensive; although they do not provide direct information about immunization coverage, they can be used to identify neighborhoods at high risk for underimmunization.
Information about immunization coverage comes from five major sources: the National Immunization Survey (NIS), retrospective school entry surveys, special area and population surveys, Clinical Assessment Software Application (CASA) surveys of clinics and private practices, and reports from managed care plans on coverage for children in their care. The NIS and retrospective kindergarten surveys estimate the coverage of the population in a given geographical area. In contrast, the CASA and managed care assessments estimate coverage levels for particular entities responsible for the children’s care (health care providers in the former case and managed care plans in the latter).
Differences in the way immunization coverage is measured in various settings—differences in samples, antigens, and time periods, for example—inhibit comparisons across managed care plans, clinics, and private physicians’ offices. While some differences may be unavoidable, opportunities for greater comparability may be achieved through technical assistance and program leadership. Understanding the nature and origins of the differences is important if remedies to improve the current situation