Evolution and polymorphism of poliovirus genomes.

The three poliovirus serotypes are very stable. Breakthrough of the serotype barrier has never been observed in the natural evolution of poliovirus. This serotype stability contrasts with the high level of genomic and phenotypic variability that occurs within the bounds of serotype. The efficient control of poliomyelitis by immunization is based upon type-specific immunity and serotype stability. The development of attenuated strains by Albert Sabin was possible because of the high variability of poliovirus genomes. The three Sabin strains, one for each serotype, were selected as variants of non-attenuated wild polioviruses, and each represents a unique poliovirus genotype. A consequence of poliovirus variability is the polymorphic character of its genome. This polymorphism makes possible the identification of poliovirus genotypes upon which studies on poliovirus evolution, virologic surveillance, and poliomyelitis diagnostics are based. The antigenic and genomic peculiarities of the Sabin strains are used to distinguish them from wild polioviruses among field isolates. The mechanisms of poliovirus variation and their significance to the evolution of both wild and vaccine poliovirus strains are the subjects of this article. The natural evolution of polioviruses is discussed in the context of the global initiative to eradicate poliomyelitis, which relies on the worldwide use of Sabin's vaccine.