A Sabin vaccine-derived field isolate of poliovirus type 1 displaying aberrant phenotypic and genetic features, including a deletion in antigenic site 1

Poliovirus strains derived from the oral poliovirus vaccine (Sabin) can be differentiated from wild-type poliovirus by tests based on either immunolog ical or genetic properties of the strains. The characterization of a recent ly identified poliovirus type 1 isolate with exceptional properties is desc ribed. Initial phenotypic analysis of the virus by use of polyclonal absorb ed antisera suggested a wild-type character. However, the different genomic analyses all confirmed the Sabin-derived character of the virus, All 17 pl aques isolated from the strain shared these properties, thus excluding the possibility of a mixture of a wild-type and a Sabin-derived strain. To eluc idate 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 ob served in regions encoding three major antigenic sites; the deduced amino a cid substitutions confirmed the aberrant results of micro-neutralization as says with site-specific monoclonal antibodies, The most striking feature wa s the existence of a hexanucleotide deletion in the VP1 gene, which gave ri se 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 stand ard conditions. Likewise, replication of the virus under cell culture condi tions was not affected by these mutations or by the deletion. Standard poli o vaccination protects against this aberrant virus, and its epidemiological significance remains open.