Immunization against poliomyelitis: risk/benefit/cost in a changing context.

Benefit/risk factors in immunization against poliomyelitis are examined from the viewpoint of the relative risk and efficacy of killed and live poliovirus vaccines in the currently changing contexts of poliomyelitis prevalence in developed and developing countries. Risk factors include virus of vaccine origin gaining access to the CNS, and failure of the vaccine to immunize. Data are presented to illustrate the degree to which the respective risks occur in developed and developing countries. The ultimate elimination of risk by eradication of wild and vaccine virus from the population is discussed.