OPV consists of live attenuated viruses that multiply in the intestinal tract and that can revert to a more virulent form, causing disease. A vaccine recipient excretes live virus for several weeks, and recipients or contacts may become infected with the virus.

Since the 1960s there have been about 100 studies reporting individual cases, case series, and national surveillances of vaccine-associated cases of paralytic poliomyelitis. Case definitions have been well developed by the CDC and the World Health Organization (WHO); a case of vaccine-associated paralytic poliomyelitis is said to occur in recipients if the onset of the disease begins 7-30 days postvaccination and is said to occur in contacts of vaccine recipients if the onset of the disease begins 7-60 days after a recipient's vaccination. Laboratory tests can identify the strain of the infecting virus as a wild-type or vaccine strain of poliovirus. The cases described above in the section History of Suspected Association are typical of those that followed in the 1970s and 1980s.

WHO and CDC have used standard definitions of cases of paralytic poliomyelitis for almost 30 years, and many nations have implemented polio surveillance systems to monitor their polio immunization programs. Table 7-2 summarizes national data from the United States, England and Wales, Belgium, Spain, Romania, Hungary, Italy, and northern Greece. Despite