Multiple Genetic Changes Can Occur in the Oral Poliovaccines upon Replication in Humans

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Abstract
Poliovirus isolates of serotypes 2 and 3 from patients whose paralytic poliomyelitis cases were classified as oral vaccine-associated were analyzed by oligonucleotide mapping of the virus RNA genomes and by polyacrylamide gel electrophoresis of the virus proteins. Oligonucleotide maps of all isolates were similar to the maps of the corresponding oral vaccine strain. No 2 isolates gave identical maps. Most maps differed from that of the vaccine strain by at least 1 oligonucleotide spot. Maps of some isolates revealed numerous differences, indicating that multiple (> 100) genetic changes had occurred in the vaccine virus genomes during replication in 1 or 2 individuals. Maps of some neural tissue isolates showed minimal differences from the reference vaccine maps, raising the possibility that neurovirulence may be restored by a small number of genetic changes. For many isolates, changes were also detected in the mobilities and processing rates of the virus proteins.