During the 1990s, the U.S. federal and state governments built a dynamic and flexible immunization system that has adapted to extensive changes in the science of vaccines, in demographic patterns, and in service-delivery patterns, in places ranging from remote rural counties to densely populated metropolitan areas. This highly decentralized system is shaped by local circumstances, resources, and needs, as well as by national goals and policies. Though complex and cumbersome, the federal-state immunization partnership has demonstrated an extraordinary capacity to ensure the reliable delivery of an increasing number of vaccine antigens for an expanding range of age groups, including newborns, preschool and school-aged children, adolescents, and adults in a growing number of private and public health care settings.

At present, however, the public health infrastructure that supports the national immunization system is fragile and unstable. Three trends contribute to this instability:

  • rapid acceleration in the science of vaccine research and production,

  • increasing complexity of the health care services environment of the United States (represented by trends such as the emergence of private managed care organizations as the primary health care providers for low-income populations), and

  • recent reductions in federal immunization grants to the states (reflecting congressional responses to shifting health care roles and responsibilities within the federal government, the states, and private health care providers), which followed on the heels of dramatic increases in the early 1990s.

This instability can create pressure points and service gaps that contribute to vaccine coverage disparities and may result in outbreaks of infectious disease. The resurgence of measles in 1989–1991 in the United States, which included a series of outbreaks that contributed to 43,000 cases and more than 100 deaths, primarily among children younger than 5 years of age, is a constant reminder that the presence of vaccines alone is not sufficient to protect populations against vaccine-preventable disease. Outbreaks can emerge swiftly and unexpectedly during times of complacency if vaccines are not accessible to those who are most vulnerable to infectious disease. The absence of adequate measurement tools and appropriate community assessment studies can result in reduced vigilance within the health care system if missing data foster mistaken beliefs that national or local immunization rates are up to date.

Although record levels of immunization were achieved across the

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