Modeling scenarios for ending poliovirus transmission in Pakistan and Afghanistan

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

Continued investment in the development and application of mathematical models of poliovirus transmission, economics, and risks leads to their use in support of polio endgame strategy development and risk management policies. This study complements an earlier review covering the period 2000-2019 and discusses the evolution of studies published since 2020 by modeling groups supported by the Global Polio Eradication Initiative (GPEI) partners and others. We systematically review modeling papers published in English in peer-reviewed journals from 2020-2024.25 that focus on poliovirus transmission and health economic analyses. In spite of the long-anticipated end of poliovirus transmission and the GPEI sunset, which would lead to the end of its support for modeling, we find that the number of modeling groups supported by GPEI partners doubled and the rate of their publications increased. Modeling continued to play a role in supporting GPEI and national/regional policies, but changes in polio eradication governance, decentralized management and decision-making, and increased heterogeneity in modeling approaches and findings decreased the overall impact of modeling results. Meanwhile, the failure of the 2016 globally coordinated cessation of type 2 oral poliovirus vaccine use for preventive immunization and the introduction of new poliovirus vaccines and formulation, increased the complexity and uncertainty of poliovirus transmission and economic models and policy recommendations during this time.

Coordinated global cessation of oral poliovirus vaccine use: Options and potential consequences

Due to the very low, but nonzero, paralysis risks associated with the use of oral poliovirus vaccine (OPV), eradicating poliomyelitis requires ending all OPV use globally. The Global Polio Eradication Initiative (GPEI) coordinated cessation of Sabin type 2 OPV (OPV2 cessation) in 2016, except for emergency outbreak response. However, as of early 2023, plans for cessation of bivalent OPV (bOPV, containing types 1 and 3 OPV) remain undefined, and OPV2 use for outbreak response continues due to ongoing transmission of type 2 polioviruses and reported type 2 cases. Recent development and use of a genetically stabilized novel type 2 OPV (nOPV2) leads to additional potential vaccine options and increasing complexity in strategies for the polio endgame. Prior applications of integrated global risk, economic, and poliovirus transmission modeling consistent with GPEI strategic plans that preceded OPV2 cessation explored OPV cessation dynamics and the evaluation of options to support globally coordinated risk management efforts. The 2022-2026 GPEI strategic plan highlighted the need for early bOPV cessation planning. We review the published modeling and explore bOPV cessation immunization options as of 2022, assuming that the GPEI partners will not support restart of the use of any OPV type in routine immunization after a globally coordinated cessation of such use. We model the potential consequences of globally coordinating bOPV cessation in 2027, as anticipated in the 2022-2026 GPEI strategic plan. We do not find any options for bOPV cessation likely to succeed without a strategy of bOPV intensification to increase population immunity prior to cessation.

Outbreak management strategies for cocirculation of multiple poliovirus types

Prior modeling studies showed that current outbreak management strategies are unlikely to stop outbreaks caused by type 1 wild polioviruses (WPV1) or circulating vaccine-derived polioviruses (cVDPVs) in many areas, and suggested increased risks of outbreaks with cocirculation of more than one type of poliovirus. The surge of type 2 poliovirus transmission that began in 2019 and continues to date, in conjunction with decreases in preventive supplemental immunization activities (SIAs) for poliovirus types 1 and 3, has led to the emergence of several countries with cocirculation of more than one type of poliovirus. Response to these emerging cocirculation events is theoretically straightforward, but the different formulations, types, and inventories of oral poliovirus vaccines (OPVs) available for outbreak response present challenging practical questions. In order to demonstrate the implications of using different vaccine options and outbreak campaign strategies, we applied a transmission model to a hypothetical population with conditions similar to populations currently experiencing outbreaks of cVDPVs of both types 1 and 2. Our results suggest prevention of the largest number of paralytic cases occurs when using (1) trivalent OPV (tOPV) (or coadministering OPV formulations for all three types) until one poliovirus outbreak type dies out, followed by (2) using a type-specific OPV until the remaining poliovirus outbreak type also dies out. Using tOPV first offers a lower overall expected cost, but this option may be limited by the willingness to expose populations to type 2 Sabin OPV strains. For strategies that use type 2 novel OPV (nOPV2) concurrently administered with bivalent OPV (bOPV, containing types 1 and 3 OPV) emerges as a leading option, but questions remain about feasibility, logistics, type-specific take rates, and coadministration costs.

Outbreak response strategies with type 2-containing oral poliovirus vaccines

Despite exhaustive and fully-financed plans to manage the risks of globally coordinated cessation of oral poliovirus vaccine (OPV) containing type 2 (OPV2) prior to 2016, as of 2022, extensive, continued transmission of circulating vaccine-derived polioviruses (cVDPVs) type 2 (cVDPV2) remains. Notably, cumulative cases caused by cVDPV2 since 2016 now exceed 2,500. Earlier analyses explored the implications of using different vaccine formulations to respond to cVDPV2 outbreaks and demonstrated how different properties of novel OPV2 (nOPV2) might affect its performance compared to Sabin monovalent OPV2 (mOPV2). These prior analyses used fixed assumptions for how outbreak response would occur, but outbreak response implementation can change. We update an existing global poliovirus transmission model to explore different options for responding with different vaccines and assumptions about scope, delays, immunization intensity, target age groups, and number of rounds. Our findings suggest that in order to successfully stop all cVDPV2 transmission globally, countries and the Global Polio Eradication Initiative need to address the deficiencies in emergency outbreak response policy and implementation. The polio program must urgently act to substantially reduce response time, target larger populations - particularly in high transmission areas - and achieve high coverage with improved access to under-vaccinated subpopulations. Given the limited supplies of nOPV2 at the present, using mOPV2 intensively immediately, followed by nOPV2 intensively if needed and when sufficient quantities become available, substantially increases the probability of ending cVDPV2 transmission globally.

Modeling undetected live type 1 wild poliovirus circulation after apparent interruption of transmission: Pakistan and Afghanistan

Since 2013, wild poliovirus (WPV) transmission occurred only for type 1 (WPV1). Following several years of increasing reported incidence (2017-2019) and programmatic disruptions caused by COVID-19 (early 2020), Pakistan and Afghanistan performed a large number of supplementary immunization activities (late 2020-2021). This increased intensity of immunization, following widespread transmission, substantially decreased WPV1 cases and positive environmental samples during 2021. Modeling the potential for undetected circulation of WPV1 after apparent interruption can support regional and global decisions about certification of the eradication of indigenous WPV1 transmission. We apply a stochastic model to estimate the confidence about no circulation (CNC) of WPV1 in Pakistan and Afghanistan as a function of time since the last reported case and/or positive environmental sample. Exploration of different assumptions about surveillance quality suggests a range for CNC for WPV1 as a function of time since the last positive surveillance signal, and supports the potential use of a time with no evidence of transmission of less than 3 years as sufficient to assume die out in the context of good acute flaccid paralysis (AFP) surveillance. We show high expected CNC based on AFP surveillance data alone, even with imperfect surveillance and some use of inactivated poliovirus vaccine masking the ability of AFP surveillance to detect transmission. Ensuring high quality AFP and environmental surveillance may substantially shorten the time required to reach high CNC. The time required for high CNC depends on whether immunization activities maintain high population immunity and the quality of surveillance data.

Health economic analysis of vaccine options for the polio eradication endgame: 2022-2036

BACKGROUND

: Multiple vaccine options are available for polio prevention and risk management. Integrated global risk, economic, and poliovirus transmission modeling provides a tool to explore the dynamics of ending all use of one or more poliovirus vaccines to simplify the polio eradication endgame.

RESEARCH DESIGN AND METHODS

: With global reported cases of poliomyelitis trending higher since 2016, we apply an integrated global model to simulate prospective vaccine policies and strategies for OPV-using countries starting with initial conditions that correspond to the epidemiological poliovirus transmission situation at the beginning of 2022.

RESULTS

: Abruptly ending all OPV use in 2023 and relying only on IPV to prevent paralysis with current routine immunization coverage would lead to expected reestablished endemic transmission of poliovirus types 1 and 2, and approximately 150,000 expected cases of poliomyelitis per year. Alternatively, if OPV-using countries restart trivalent OPV (tOPV) use for all immunization activities and end IPV use, the model shows the lowest anticipated annual polio cases and lowest costs.

CONCLUSIONS

: Poor global risk management and coordination of OPV cessation remain a critical failure mode for the polio endgame, and national and global decision makers face difficult choices due to multiple available polio vaccine options and immunization strategies.