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tated the development of other live attenuated or inactivated candidate vaccines (Koprowski et al., 1952; Sabin, 1955). The inactivated type of poliovirus vaccine (IPV, also known as the “Salk vaccine”) was licensed in April 1955, and the oral, live-attenuated polio vaccine (OPV, also known as the “Sabin vaccine”) was licensed for human use in 1961–1962 (Commission on the Cost of Medical Care, 1964).

With the introduction of polio vaccines, the incidence of poliomyelitis declined sharply. In 1965, only 59 cases of poliomyelitis were reported in the United States, and continued widespread use of OPV alone has essentially eliminated polio in the Americas. The last case of indigenously acquired wildtype poliovirus infection reported in the United States occurred in 1979 (Hinman et al., 1987). In 1985, the Pan-American Health Organization (PAHO) established the goal of eliminating poliomyelitis from the Western Hemisphere. The subsequent success of their efforts is best exemplified by the fact that the last confirmed case of paralytic poliomyelitis associated with wild-type virus infection occurred in Peru in 1991.

The worldwide rate of routine immunization for polio increased from about 47% in 1985 to 80% in 1994. The annual number of cases of polio reported decreased from 39,361 in 1985 to about 6,241 in 1990, a decline of nearly 85%. Encouraged by these dramatic results, WHO established the goal of the global eradication of poliomyelitis by the year 2000 (CDC, 1995c). The worldwide eradication campaign is relying on use of OPV, but several countries in Europe successfully eradicated poliovirus with IPV only.

Advantages of Evolving Vaccine Strategies

As pointed out earlier, the large-scale use of IPV reduced the annual reported worldwide incidence of paralytic polio to about 0.8 cases per 100,000 population (about 900 cases) by 1961. At that time, more than 485 million doses of the vaccine had been distributed in the United States. Occasional cases of disease were reported in fully vaccinated subjects in the early phases of the vaccination program, so continued efforts in vaccine development led to improvements in its biologic activity and the introduction of enhanced-potency IPV (eIPV). Vaccine failures with eIPV are much reduced from the earlier version of the vaccine. No cases of paralytic disease have been reported in subjects successfully immunized with eIPV, which is extensively used as the vaccine of choice in many parts of the world.

Since 1962, the United States had depended primarily on OPV in its polio eradication efforts. Between 1969 and 1983, however, about 225 cases of paralytic poliomyelitis associated with OPV were identified in the United States. An average of 8 to 10 cases of vaccine-associated paralytic polio continued to be reported in the United States each year. Despite the success and the benefits achieved with OPV and the continued absence of wild-type virusassociated disease, the rare occurrence of vaccine-associated paralytic disease

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