Oral poliovirus vaccine: history of its development and use and current challenge to eliminate poliomyelitis from the world.

Oral poliovirus vaccine (OPV) is like no other live virus vaccine used in humans: vaccine strains multiply extensively in the intestinal tract, are widely disseminated in the family and community, and immunize a large proportion of the unvaccinated population. During the search for optimal strains for vaccine use, motor neurons in the spinal cord of chimpanzees (and by extrapolation those of humans) were found to be much more resistant to polioviruses than those of monkeys; the reverse was true for the alimentary tract. Various biologic properties of polioviruses also varied quantitatively over a wide spectrum and were genetically distinct. The phenomenon of somewhat increased neurovirulence for monkeys, but not for chimpanzees, encountered in excreted virus was extensively studied in families, in children's homes, and finally among hundreds of thousands of susceptible children and adults in areas where only 50% of the susceptible population received OPV; these studies did not reveal evidence of danger. During the past 20 years approximately 5 million cases of paralytic poliomyelitis were probably prevented by OPV in predominantly temperate-climate countries inhabited by approximately 2 billion people. OPV has also been used less extensively and not optimally in many tropical and subtropical countries, where paralytic poliomyelitis is now known to be an important public health problem, with reduction in numbers of cases but not elimination of the disease except in some countries with better health services. Experience in Cuba during the past 21 years, in Brazil during the past 5 years, and in the Dominican Republic during the past 2 years has shown that the strategy of annual short-term vaccination of all children in the most susceptible age groups can rapidly eliminate the disease from tropical and subtropical countries.