Retrospective molecular and phenotypic analysis of poliovirus vaccine strains isolated in Greece

The live oral poliovirus vaccine (OPV) strains are genetically unstable, causing, in rare cases, vaccine-associated paralytic poliomyelitis. Reversions of the known attenuating mutations in OPV strains and intertypic recombination have been identified as the underlying causes of the increased neurovirulence of poliovirus isolates. In this study, three OPV isolates (one non-recombinant and two recombinants) were tested in order to correlate phenotypic traits such as temperature sensitivity (Rct test) and growth kinetics (one-step growth curve test) with mutations and recombination events of the viral genome. Moreover, the immunity level of the western Greek population aged 1-40 years was evaluated against OPV isolates and Sabin vaccine strains, with a microneutralization assay. Members of the 1-40-year age group (both pooled and individual sera) showed no significant differences in neutralization test (NT) titres against OPV isolates in comparison with the Sabin vaccine strains. However, all three OPV isolates showed reverted phenotypic traits in Rct or one-step growth curve assays. The results of our study revealed a significant decrease in immunity level from the 1-10-year age group to the 21-30-year age group (pooled sera) for both poliovirus types 1 and 3. For both poliovirus types, the highest NT titres were observed in the 1-10-year age group, and the lowest NT titre was observed in the 21-30-year age group, towards poliovirus type 3. Our study underlines the need for immunological studies in all age groups, in order to allow reconsideration of the current vaccination policies and to avoid epidemics caused by the circulation of highly evolved OPV derivatives.