Vaccination can change the ecology of infectious disease in human populations. It converts the equation of transmissibility in susceptible populations to predictable calculations of the length of time that an infectious agent may survive in immunized communities. But the potential eradication or control of many infectious diseases is threatened by a staggering disparity in vaccination efforts worldwide. The successes of the 20th-century global immunization programs have been significant: the eradication of smallpox and the elimination of poliomyelitis in Europe, the Americas, and much of Asia. Yet a huge gap exists between the industrialized world and sub-Saharan Africa in the immunization rates for diphtheria, tetanus, and pertussis (DTP), the most accessible and affordable childhood vaccine. The DTP vaccination rate on the African continent has hovered around 50% for the past 15 years, with approximately one-quarter of the countries in the region falling below this average. The persistence of this gap is an outcome of unsustairied im- munization efforts along with a deteriorating infrastructure and lack of political will. Furthermore, immunization rates of other vaccines, such as that for hepatitis B, are far below that for DTP.
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