Evolving epidemiology of poliovirus serotype 2 following withdrawal of the serotype 2 oral poliovirus vaccine.

Although there have been no cases of serotype 2 wild poliovirus for more than 20 years, transmission of serotype 2 vaccine-derived poliovirus (VDPV2) and associated paralytic cases in several continents represent a threat to eradication. The withdrawal of the serotype 2 component of oral poliovirus vaccine (OPV2) was implemented in April 2016 to stop VDPV2 emergence and secure eradication of all serotype 2 poliovirus. Globally, children born after this date have limited immunity to prevent transmission. Using a statistical model, we estimated the emergence date and source of VDPV2s detected between May 2016 and November 2019. Outbreak response campaigns with monovalent OPV2 are the only available method to induce immunity to prevent transmission. Yet our analysis shows that using monovalent OPV2 is generating more paralytic VDPV2 outbreaks with the potential for establishing endemic transmission. A novel OPV2, for which two candidates are currently in clinical trials, is urgently required, together with a contingency strategy if this vaccine does not materialize or perform as anticipated.