Modeling Undetected Live Poliovirus Circulation After Apparent Interruption of Transmission: Borno and Yobe in Northeast Nigeria.

Silent circulation of polioviruses complicates the polio endgame by affecting the confidence with which we can certify successful eradication (i.e., the end of transmission everywhere) given a long enough period of time with active surveillance and no observed detections. The Global Polio Eradication Initiative continues to use three years without observing paralytic cases caused by wild poliovirus (WPV) infection as an indication of sufficient confidence that poliovirus circulation stopped (assuming good surveillance). Prior modeling demonstrated the complexities of real populations and the imperfect nature of real surveillance systems, and highlighted the need for modeling the specific last reservoirs of undetected circulation. We use a poliovirus transmission model developed for Borno and Yobe to characterize the probability of undetected poliovirus circulation once apparent die-out occurs (i.e., in the absence of epidemiological signals) for WPV serotypes 1 and 3. Specifically, we convert the model to a stochastic form that supports estimates of confidence about no circulation given the time since the last detected event and considering the quality of both immunization and surveillance activities for these states. We find high confidence of no WPV3 circulation, and increasing confidence of WPV1 circulation, which we anticipate will imply high confidence in the absence of any detected cases in mid-2020 so long as Borno and Yobe maintain similar or achieve improved conditions. Our results confirm that gaps in poliovirus surveillance or reaching elimination with borderline sufficient population immunity can substantially increase the time to reach a high confidence about no undetected poliovirus transmission.