Frequent isolation of intertypic poliovirus recombinants with serotype 2 specificity from vaccine‐associated polio cases

Abstract
Five representatives from a collection of 21 Sabin type 2-like poliovirus strains isolated from paralytic poliomyelitis cases in two regions of the USSR have been subjected t o limited nucleotide sequencing. All proved to be intertypic recombinants having the genes encoding capsid proteins of Sabin 2 origin and a 3'-end portion of the genome derived from either type 1 (3 isolates) or type 3 (2 isolates) Sabin strains. The crossover points in all the 5 genomes have been mapped to different loci of the P3 region. At least 6 additional isolates from the same collection (and 2 isolates from healthy contacts), appeared to have a type 2/type 1 recombinant genome, as judged by oligonucleotide mapping. The biological significance of frequent occurrence of recombinants among field isolates of vaccine-related strains is discussed.