Type 2 poliovirus detection after global withdrawal of trivalent oral vaccine

BACKGROUND: Mass campaigns with oral poliovirus vaccine (OPV) have brought the world close to wild-type poliovirus eradication. However, to complete eradication, OPV must itself be withdrawn to prevent vaccine-derived poliovirus outbreaks. Synchronized global withdrawal of OPV began with serotype-2 (OPV2) in April 2016, presenting the first test of the feasibility of eradicating all polioviruses. METHODS: We analysed global surveillance data reporting serotype-2 vaccine poliovirus (Sabin-2) and vaccine- derived poliovirus (VDPV2) detection in stool collected during 1 January 2013 through 08 August 2017 from 431,429 children with acute flaccid paralysis in 112 countries and 5485 environmental samples from 4 high-risk countries. We used Bayesian spatiotemporal smoothing and logistic regression to identify and map risk-factors for persistent Sabin-2 and VDPV2 detection. RESULTS: Detection of Sabin-2 declined rapidly from 3.9% in stool [95% Confidence Interval: 3.5%-4.3%] and 71% [61%-80%] in sewage at the time of OPV2 withdrawal to 0.16% [0.09%-2.7%] and 13% [8%- 20%] at 2 months. However, at 12 months Sabin-2 continued to be detected (0.05% [0.01%-0.13%] in stool, 8% [5%-13%] in sewage) due to OPV2 use in response to VDPV2 outbreaks. Five outbreaks were reported after OPV2 withdrawal, associated with low routine immunisation coverage and population immunity (Odds Ratios 2.59 [1.26-6.33] and 4.65 [1.71-15.28] per 10% absolute decrease). DISCUSSION: High population immunity has facilitated rapid decline of Sabin-2 after OPV2 withdrawal and restricted circulation of VDPV2 to known areas at high risk of transmission. It will be critical to control these remaining VDPV2 outbreaks before the growth of significant cohorts of susceptible children.