Poliovirus strains derived from the oral poliovirus vaccine (Sabin) can be differentiated from wild-type poliovirus by tests based on either immunological or genetic properties of the strains. The characterization of a recently identified poliovirus type 1 isolate with exceptional properties is described. Initial phenotypic analysis of the virus by use of polyclonal absorbed antisera suggested a wild-type character. However, the different genomic analyses all confirmed the Sabin-derived character of the virus. All 17 plaques isolated from the strain shared these properties, thus excluding the possibility of a mixture of a wild-type and a Sabin-derived strain. To elucidate the properties of this virus further, the nucleotide sequences of the P1 region and most of the 5' non-coding region were established. Although the nucleotide identity with Sabin 1 was more than 99.4%, mutations were observed in regions encoding three major antigenic sites; the deduced amino acid substitutions confirmed the aberrant results of micro-neutralization assays with site-specific monoclonal antibodies. The most striking feature was the existence of a hexanucleotide deletion in the VP1 gene, which gave rise to a two amino acid deletion in the BC loop. In spite of these antigenic changes, the strain was readily serotyped as poliovirus type 1 under standard conditions. Likewise, replication of the virus under cell culture conditions was not affected by these mutations or by the deletion. Standard polio vaccination protects against this aberrant virus, and its epidemiological significance remains open.


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