Looking back at prospective modeling of outbreak response strategies for managing global type 2 oral poliovirus vaccine (OPV2) cessation

Abstract

Introduction: Detection of poliovirus transmission and ongoing oral poliovirus vaccine (OPV) use continue to delay poliomyelitis eradication. In 2016, the Global Polio Eradication Initiative (GPEI) coordinated global cessation of type 2 OPV (OPV2) for preventive immunization and limited its use to emergency outbreak response. In 2019, GPEI partners requested restart of some Sabin OPV2 production and also accelerated the development of a genetically modified novel OPV2 vaccine (nOPV2) that promised greater genetic stability than monovalent Sabin OPV2 (mOPV2).

Methods: We reviewed integrated risk, economic, and global poliovirus transmission modeling performed before OPV2 cessation, which recommended multiple risk management strategies to increase the chances of successfully ending all transmission of type 2 live polioviruses. Following OPV2 cessation, strategies implemented by countries and the GPEI deviated from model recommended risk management strategies. Complementing other modeling that explores prospective outbreak response options for improving outcomes for the current polio endgame trajectory, in this study we roll back the clock to 2017 and explore counterfactual trajectories that the polio endgame could have followed if GPEI had: (1) managed risks differently after OPV2 cessation and/or (2) developed nOPV2 before and used it exclusively for outbreak response after OPV2 cessation.

Results: The implementation of the 2016 model-based recommended outbreak response strategies could have ended (and could still substantially improve the probability of ending) type 2 poliovirus transmission. Outbreak response performance observed since 2016 would not have been expected to achieve OPV2 cessation with high confidence, even with the availability of nOPV2 prior to the 2016 OPV2 cessation.

Discussion: As implemented, the 2016 OPV2 cessation failed to stop type 2 transmission. While nOPV2 offers benefits of lower risk of seeding additional outbreaks, its reduced secondary spread relative to mOPV2 may imply relatively higher coverage needed for nOPV2 than mOPV2 to stop outbreaks.

Keywords: cessation; dynamic modeling; eradication; immunization; oral poliovirus vaccine; outbreak response; polio.

Figure 1

Similarity between the expected global number of paralytic WPV1 (left panel) and cVDPV2 (right panel) cases by year for 100 stochastic iterations of the “mOPV2, current oSIA” scenario for 2016–2021 (dashed lines) compared to the deterministic reference case (RC, solid lines). In all modeled scenarios, IPV is used in RI in all countries as a prerequisite to OPV cessation. The actual number of reported cases are shown in solid bars for reference.

Figure 2

Modeled expected global number of paralytic cVDPV2 cases by year for 100 stochastic iterations of the different vaccine choices and oSIA characteristics for the period 2016–2026. The figure legend shows outbreak vaccine choice and response characteristics. The panel on the left shows results for modeled scenarios in which IPV is used in RI in all countries as a prerequisite to OPV cessation. The panel on the right shows results for modeled scenarios in which there is no prerequisite for introduction of IPV into RI in countries classified as low-income or lower middle-income by the World Bank.