Reflections on Modeling Poliovirus Transmission and the Polio Eradication Endgame

Abstract

The Global Polio Eradication Initiative (GPEI) partners engaged modelers during the past nearly 20 years to support strategy and policy discussions and decisions, and to provide estimates of the risks, costs, and benefits of different options for managing the polio endgame. Limited efforts to date provided insights related to the validation of the models used for GPEI strategy and policy decisions. However, modeling results only influenced decisions in some cases, with other factors carrying more weight in many key decisions. In addition, the results from multiple modeling groups do not always agree, which supports selection of some strategies and/or policies counter to the recommendations from some modelers but not others. This analysis reflects on our modeling, and summarizes our premises and recommendations, the outcomes of these recommendations, and the implications of key limitations of models with respect to polio endgame strategy. We briefly review the current state of the GPEI given epidemiological experience as of early 2020, which includes failure of the GPEI to deliver on the objectives of its 2013–2018 strategic plan despite full financial support. Looking ahead, we provide context for why the GPEI strategy of global oral poliovirus vaccine (OPV) cessation to end all cases of poliomyelitis looks infeasible given the current state of the GPEI and the failure to successfully stop all transmission of serotype 2 live polioviruses within four years of the April–May 2016 coordinated cessation of serotype 2 OPV use in routine immunization.

Review of poliovirus modeling performed from 2000 to 2019 to support global polio eradication

“…Although outside of the time window for this review, in early 2020, KRI published an updated version of its integrated model to account for the programmatic experience, vaccination achieved, and epidemiology through 2019 [ 202 ]. This process included updating the inputs for its iVDPV risk model [ 81 ], and focused on actual and expected performance throughout the polio endgame instead of assuming optimistic and ideal risk management from 2015 on [ 203 ] as KRI assumed earlier [ 51 ].…”

“…KRI suggests that chasing better (and often more expensive) tools (e.g. mOPV, IPV) has not helped accelerate global polio eradication, that achieving and maintaining eradication depends on continuing to get enough OPV preventively into susceptible children to stop and prevent the transmission of cVDPVs and/or WPVs (followed by careful and aggressive management of the risks of globally coordinated OPV cessation), and that as of early 2020, the GPEI appears off track [ 202 , 203 ].…”

“…All models depend on the scope (i.e. boundaries of the system), assumptions about structure of the system and the causal relationships that determine the equations used and the selection of model inputs, and are limited by their assumptions [ 203 ]. This section highlights some of the key differences in and limitations of the different modeling approaches.…”

Updated Characterization of Post‐OPV Cessation Risks: Lessons from 2019 Serotype 2 Outbreaks and Implications for the Probability of OPV Restart

“…The application of the updated risks in global modeling can provide a baseline for the global risks of OPV2 restart in the absence of nOPV2, or if the unknown behavior of nOPV2 matches the behavior of mOPV2. Prior to OPV2 cessation, we made optimistic assumptions about future GPEI and national programmatic performance compared to now observed actual performance (Thompson & Kalkowska, 2020), and estimated a global risk of any OPV restart (for any serotype) on the order of approximately 5–6% for 2013–2052 (Duintjer Tebbens, Pallansch, Wassalik, Cochi, & Thompson, 2015). Because of the risks of using mOPV2 in populations that would increasingly become vulnerable to the introductions of mOPV2 leading to unstoppable transmission (Duintjer Tebbens et al., 2016a), our earlier modeling also demonstrated the importance of: (i) achieving and maintaining high‐quality surveillance to quickly detect any transmission, (ii) managing global OPV2 cessation to end all LPV2 infections as quickly as possible by aggressively responding to and shutting down any outbreaks using mOPV2, and (iii) developing of a global stockpile of mOPV2 to enable rapid and aggressive response (Duintjer Tebbens, Pallansch, Wassilak, Cochi, & Thompson, 2016 ; Duintjer Tebbens, Pallansch, Alexander, & Thompson, 2010; Duintjer Tebbens & Thompson, 2017, 2018; Thompson & Duintjer Tebbens, 2008).…”

Potential Future Use, Costs, and Value of Poliovirus Vaccines

“…Prior to the Coronavirus Disease 2019 (COVID‐19) pandemic, the risk of insufficient support for the GPEI (i.e., financial risks) posed a threat to the GPEI and polio eradication. Despite full support from donors of its 2013–2018 Strategic Plan (World Health Organization Global Polio Eradication Initiative, 2013), the GPEI did not deliver on achieving polio eradication by 2020 (Thompson & Kalkowska, 2020). The GPEI released a 2019–2023 Strategic Plan with a $4.2 billion budget (World Health Organization Global Polio Eradication Initiative, 2019), but financial risks remain a concern, and the COVID‐19 pandemic will likely further delay eradication and increase costs.…”