Evolution of poliovirus since introduction of attenuated vaccine.

Genetic marker tests were performed on 997 strains of poliovirus isolated from patients with neurological disease and from healthy people in England and Wales. Before the introduction of live attenuated vaccine most strains could multiply at raised temperatures. Now, however, many strains isolated from cases of poliomyelitis or from healthy persons with no known contact with vaccine cannot grow above 37 degrees C, and in this respect resemble the vaccine strains. The three serotypes are also much more evenly represented. Hence probably to a limited extent the vaccine-like strains have established themselves in the community.