Has Wild Poliovirus Been Eliminated from Nigeria?
Wild poliovirus type 3 (WPV3) has not been seen anywhere since the last case of WPV3-associated paralysis in Nigeria in November 2012. At the time of writing, the most recent case of wild poliovirus type 1 (WPV1) in Nigeria occurred in July 2014, and WPV1 has not been seen in Africa since a case in Somalia in August 2014. No cases associated with circulating vaccine-derived type 2 poliovirus (cVDPV2) have been detected in Nigeria since November 2014. Has WPV1 been eliminated from Africa? Has WPV3 been eradicated globally? Has Nigeria interrupted cVDPV2 transmission? These questions are difficult because polio surveillance is based on paralysis and paralysis only occurs in a small fraction of infections. This report provides estimates for the probabilities of poliovirus elimination in Nigeria given available data as of March 31, 2015. It is based on a model of disease transmission that is built from historical polio incidence rates and is designed to represent the uncertainties in transmission dynamics and poliovirus detection that are fundamental to interpreting long time periods without cases. The model estimates that, as of March 31, 2015, the probability of WPV1 elimination in Nigeria is 84%, and that if WPV1 has not been eliminated, a new case will be detected with 99% probability by the end of 2015. The probability of WPV3 elimination (and thus global eradication) is > 99%. However, it is unlikely that the ongoing transmission of cVDPV2 has been interrupted; the probability of cVDPV2 elimination rises to 83% if no new cases are detected by April 2016.
Figure 1: Model results for elimination and time to next case
Nigeria hasn't detected any wild poliovirus since July 2014. But polio is not easy to detect. How likely is it that wild poliovirus is gone?
Probability of elimination assuming no new cases are observed: WPV1 (A), WPV3 (C), cVDPV2 (E). Probability of observing a new case if elimination doesn’t occur, WPV1 (B), WPV3 (D), cVDPV2 (F). Fig prepared March 31, 2015 (black dashed line). In each panel, the horizontal axis origin is the time of the most recent case, the solid blue curve shows the scenario with mean Reff = 1 and perfect surveillance, the green curve shows a less conservative scenario with mean Reff < 1 and perfect surveillance, and the dashed blue curve shows a conservative scenario with mean Reff = 1 and 50% surveillance sensitivity. For cVDPV2 (C,F), the orange curve depicts an optimistic scenario in which the mean Reff is held at the lowest value ever observed and the standard deviation is reduced to one-fourth of its observed value. doi:10.1371/journal.pone.0135765.g001
After submission, on June 26, a cVDPV2 case in Nigeria was confirmed by the Global Polio Laboratory Network. The case provides information about cVDPV2 prevalence that is useful for assessing the accuracy of the predictions in this paper and for updating projections for cVDPV2 persistence and elimination. An expanded version of this paper that incorporates the new information is archived at http://arxiv.org/abs/1504.02751.