Key result
Confirming polio interruption requires up to ~3 years without cases, with poor surveillance adding two-plus years.
Sustaining high-quality surveillance is critical in Pakistan and Afghanistan to ensure true interruption of poliovirus transmission and avoid delaying the global polio endgame.
Extended surveillance required years after last case in endemic areas; modeling leaves open optimal certification timelines for eradication.
Since most poliovirus infections occur with no paralytic symptoms, the possibility of silent circulation complicates the confirmation of the end of poliovirus transmission. Based on empirical field experience and theoretical modeling results, the Global Polio Eradication Initiative identified three years without observing paralytic cases from wild polioviruses with good acute flaccid paralysis surveillance as an indication of sufficient confidence that poliovirus circulation stopped. The complexities of real populations and the imperfect nature of real surveillance systems subsequently demonstrated the importance of specific modeling for areas at high risk of undetected circulation, resulting in varying periods of time required to obtain the same level of confidence about no undetected circulation. Using a poliovirus transmission model that accounts for variability in transmissibility and neurovirulence for different poliovirus serotypes and characterizes country-specific factors (e.g., vaccination and surveillance activities, demographics) related to wild and vaccine-derived poliovirus transmission in Pakistan and Afghanistan, we consider the probability of undetected poliovirus circulation for those countries once apparent die-out occurs (i.e., in the absence of any epidemiological signals). We find that gaps in poliovirus surveillance or reaching elimination with borderline sufficient population immunity could significantly increase the time to reach high confidence about interruption of live poliovirus transmission, such that the path taken to achieve and maintain poliovirus elimination matters. Pakistan and Afghanistan will need to sustain high-quality surveillance for polioviruses after apparent interruption of transmission and recognize that as efforts to identify cases or circulating live polioviruses decrease, the risks of undetected circulation increase and significantly delay the global polio endgame.
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Kalkowska et al. (2018) studied Poliovirus. Poliovirus surveillance systems (Acute Flaccid Paralysis and Environmental Surveillance) vs. Perfect vs imperfect surveillance was evaluated on Time required to achieve 95% confidence about no circulation (CNC95%). Modeling in Pakistan and Afghanistan indicates that achieving 95% confidence in transmission interruption requires 1.4 to 2.6 years without cases under perfect surveillance, with poor surveillance delaying this by over two years.
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