Oral poliovaccine: will it help eradicate polio or cause the next epidemic?

Background: Poliovirus rapidly evolves by nucleic acid substitutions and genetic recombination with other polioviruses and non-polio enteroviruses. Evolving oral poliovaccine can rapidly revert to neurovirulence and undergo antigenic alterations.

Objectives: To evaluate the threat of vaccine-derived poliovirus (1-15% divergence from the respective Sabin strain) for a poliomyelitis-free population in a country with a long-standing routine vaccination program.

Methods: We characterized genetic and antigenic changes in OPV (Sabin) strains isolated from sewage in Israel and evaluated intestinal immunity by measuring fecal excretion after OPV challenge of vaccinated children.

Results: Characterization of poliovirus from sewage revealed eight type 2 and three type 3 vaccine polioviruses that had replicated and started to evolve (vaccine that replicated and diverged by 0.5 to < or = 1.0%) and nine highly diverged type 2 vaccine-derived polioviruses (1-15% divergence from the respective Sabin strain) with 8-14% divergence between the years 1998 and 2005. Six of the eleven VRPV uniquely recombined with OPV and/or NPEV. The nine VDPV were epidemically related, genotypically neurovirulent, and had 10-15 amino acid substitutions in antigenic sites altering their antigenicity, but shared a single recombination. Type 2 OPV was excreted by 23% and 17% of infants challenged with OPV 3 months after partial immunization (two doses each of OPV and enhanced inactivated poliovirus) or full immunization (three doses of each) respectively, despite high humoral antibody titers.

Conclusions: Our findings, which show that OPV is excreted for a significant period by children with high humoral immunity, emphasize the long-term potential threat from VDPV in highly vaccinated populations. An adequate immunization program, combined with environmental surveillance, is necessary to prevent poliomyelitis and community transmission of poliovirus.