Immunizing the U.S. migrant population presents special challenges as well. Approximately 750,000 migrants live in the United States (Mountain, 1999). Since many migrants cross state borders, they are literally a moving target for health financing, service delivery, and state-based surveillance systems. In addition, a considerable number of migrants may be undocumented persons, creating ethical and political dilemmas regarding the financing of their immunizations (Mountain, 1999).

In addition to immigration and migration, the aging of the U.S. population merits consideration in the development of strategies for immunization policy and practice. As larger numbers of individuals enjoy increased life spans, the importance of vaccines such as influenza and pneumococcal will increase.

The Challenge of Change

The above changes in vaccine development, the nature of disease, and population demographics create challenges for the U.S. immunization system. First, delays and gaps in the uptake of new vaccines occur. For example, the negotiation of a federal contract price with manufacturers of varicella vaccine required 1 year, causing significant delays in the availability of publicly purchased varicella vaccine following its appearance on the market (N.Smith, CDC, personal communication, February 10, 2000). Even with this major financial barrier removed, the national pediatric coverage rate for varicella was only 43.2 percent in 1998 (CDC, 1998a). Second, the addition of new vaccines to the schedule has broadened discrepancies among state standards and coverage practices. State immunization requirements for school children vary considerably (see Appendix G). In addition, some states mandate insurance coverage of pediatric immunizations, but the policies affected and specific vaccines covered differ greatly (Freed et al., 1999). Third, a more complex immunization schedule has made it more difficult to confirm the immunization status of special groups. Identifying pockets of need has become problematic because records are scattered among public and private providers even as the number of vaccines that require surveillance has increased. Finally, the dramatic, almost exponential increase in vaccines on the horizon creates concerns about adverse reactions. As the general public becomes less familiar with the nature and threat of infectious disease, reports of adverse events associated with the use of vaccines are likely to acquire greater significance.

Finding 2–1. The rate of change in the immunization schedule, the nature of infectious disease, and population demographics increased dramatically in the 1990s and is likely to accelerate in the future. At the same



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