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.
Added July 10, 2015: On June 26, a cVDPV2 case was confirmed by the Global
Polio Laboratory Network. The date of paralysis was May 16. The case provides
new information about cVDPV2 prevalence that is useful for assessing the
accuracy of previous predictions and informing an updated forecast for the time
to cVDPV2 elimination.Comment: Added model validation section and updated cVDPV2 forecast in
response to new case data; expanded material on surveillance sensitivity;
additional minor edits; and references added. 24 pages, 4 figure
