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

Abstract

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.

Fig. 1.. Geographic location of vaccine-derived poliovirus serotype 2 (VDPV2) isolates detected between 1 May 2016 and 1 November 2019.

The color of points illustrates the date of isolate detection. Data are as of 1 November 2019.

Fig. 2.. Incidence of vaccine-derived poliovirus serotype 2 (VDPV2) isolates detected between 1 May 2016 and 1 November 2019.

(A) The probability that isolate was seeded after the Switch (1 May 2016) was calculated according to the 95% confidence intervals of the estimated seeding date, estimated by the number of nucleotides of divergence from the poliovirus vaccine strain, in the viral protein 1 gene of the position, assuming a model for the mutation rate (see supplementary materials). (B) For all isolates with >0.9 probability of post-Switch seeding, the color demonstrates whether there was a corresponding mOPV2 campaign within estimated dates of seeding and the same or adjacent country.

Fig. 3.. Timeline of circulating VDPV2 outbreaks reported between 1 May 2016 and 1 November 2019, ordered by the date of first isolate detection.

The estimated seeding date (i.e., the date that an infectious OPV dose was administered) and 95% confidence intervals are given by horizontal bars, colored by the probability that the date of seeding was after the Switch on 1 May 2016 (dashed black line denotes date of Switch). Detected virus isolates are shown by colored circles, with the color indicating whether the outbreak is assumed to be active (detection within previous 12 months) or closed (no detection in previous 12 months). Data are as of 1 November 2019. NIE-BOS-16: This outbreak was genetically linked to a cVDPV2 emergence originating in Chad in 2012.