The Origins and Risk Factors for Serotype-2 Vaccine-Derived Poliovirus Emergences in Africa During 2016–2019

Serotype 2 oral poliovirus vaccine (OPV2) can revert to regain wild-type neurovirulence and spread to cause emergences of vaccine-derived poliovirus (VDPV2). After its global withdrawal from routine immunization in 2016, outbreak response use has created a cycle of VDPV2 emergences that threaten eradication. We implemented a hierarchical model based on VP1 region genetic divergence, time, and location to attribute emergences to campaigns and identify risk factors. We found that a 10 percentage point increase in population immunity in children younger than 5 years at the campaign time and location corresponds to a 18.0% decrease (95% credible interval [CrI], 6.3%–28%) in per-campaign relative risk, and that campaign size is associated with emergence risk (relative risk scaling with population size to a power of 0.80; 95% CrI, .50–1.10). Our results imply how Sabin OPV2 can be used alongside the genetically stable but supply-limited novel OPV2 (listed for emergency use in November 2020) to minimize emergence risk.