Worst‐case scenarios: Modeling uncontrolled type 2 polio transmission

Abstract

In May 2016, the Global Polio Eradication Initiative (GPEI) coordinated the cessation of all use of type 2 oral poliovirus vaccine (OPV2), except for emergency outbreak response. Since then, paralytic polio cases caused by type 2 vaccine‐derived polioviruses now exceed 3,000 cases reported by 39 countries. In 2022 (as of April 25, 2023), 20 countries reported detection of cases and nine other countries reported environmental surveillance detection, but no reported cases. Recent development of a genetically modified novel type 2 OPV (nOPV2) may help curb the generation of neurovirulent vaccine‐derived strains; its use since 2021 under Emergency Use Listing is limited to outbreak response activities. Prior modeling studies showed that the expected trajectory for global type 2 viruses does not appear headed toward eradication, even with the best possible properties of nOPV2 assuming current outbreak response performance. Continued persistence of type 2 poliovirus transmission exposes the world to the risks of potentially high‐consequence events such as the importation of virus into high‐transmission areas of India or Bangladesh. Building on prior polio endgame modeling and assuming current national and GPEI outbreak response performance, we show no probability of successfully eradicating type 2 polioviruses in the near term regardless of vaccine choice. We also demonstrate the possible worst‐case scenarios could result in rapid expansion of paralytic cases and preclude the goal of permanently ending all cases of poliomyelitis in the foreseeable future. Avoiding such catastrophic scenarios will depend on the development of strategies that raise population immunity to type 2 polioviruses.

Review of Poliovirus Transmission and Economic Modeling to Support Global Polio Eradication: 2020–2024

“…This section begins with a brief overview of each of the 102 studies identified by the search process summarized in Figure 1 [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 ,…”

“…With ongoing discussions about potential global coordination of bOPV cessation, KRI also applied its global integrated model to characterize the expected outcomes of coordinated global cessation of bOPV use in 2027 without bOPV pSIAs [ 41 ], as suggested by the 2022–2026 strategic plan [ 176 ]. In addition to exploring the expected annual paralytic cases by type over time, KRI also explored the worst-case scenarios [ 42 ]. In the context of low expectations of successful bOPV cessation, a related health economic analysis anticipated a relatively low value of antiviral drugs [ 43 ].…”

“…The bottom portion of Table 2 shows the geographic scope covered by 45 studies that included poliovirus transmission modeling [ 7 , 9 , 10 , 11 , 12 , 14 , 15 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 25 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 67 , 79 , 80 , 81 , 87 , 88 , 89 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 ]. Not surprisingly, two groups published studies on Pakistan, Afghanistan and Nigeria, which continue to represent critical geographies for polio eradication.…”

Health Economic Analysis of Antiviral Drugs in the Global Polio Eradication Endgame

“… 37 This rapid increase in paralytic cases occurs due to the high transmissibility and neurovirulence of type 1 polioviruses, with the expected value increased by worst case iterations in which transmission occurs in countries with high transmission potential and/or low coverage. 57 Although countries and the GPEI could potentially manage bOPV cessation to minimize and potentially eliminate the risks of cVDPV1 cases, 58 , 59 current GPEI plans appear poised to repeat the experience of insufficient population immunity to transmission prior to OPV cessation that occurred with OPV2 cessation. 37 Table 2 summarizes the results from the 100 iterations for the number of iVDPV introductions by type, with 53 total expected iVDPV introductions on average for the 100 iterations the No PAVD scenario, and Table 3 summarizes the results for the numbers of cases by type, with 162 total expected cases on average for the 100 iterations of the No PAVD scenario.…”