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

Abstract

After the globally coordinated cessation of any serotype of oral poliovirus vaccine (OPV), some risks remain from undetected, existing homotypic OPV‐related transmission and/or restarting transmission due to several possible reintroduction risks. The Global Polio Eradication Initiative (GPEI) coordinated global cessation of serotype 2‐containing OPV (OPV2) in 2016. Following OPV2 cessation, the GPEI and countries implemented activities to withdraw all the remaining trivalent OPV, which contains all three poliovirus serotypes (i.e., 1, 2, and 3), from the supply chain and replace it with bivalent OPV (containing only serotypes 1 and 3). However, as of early 2020, monovalent OPV2 use for outbreak response continues in many countries. In addition, outbreaks observed in 2019 demonstrated evidence of different types of risks than previously modeled. We briefly review the 2019 epidemiological experience with serotype 2 live poliovirus outbreaks and propose a new risk for unexpected OPV introduction for inclusion in global modeling of OPV cessation. Using an updated model of global poliovirus transmission and OPV evolution with and without consideration of this new risk, we explore the implications of the current global situation with respect to the likely need to restart preventive use of OPV2 in OPV‐using countries. Simulation results without this new risk suggest OPV2 restart will likely need to occur (81% of 100 iterations) to manage the polio endgame based on the GPEI performance to date with existing vaccine tools, and with the new risk of unexpected OPV introduction the expected OPV2 restart probability increases to 89%. Contingency planning requires new OPV2 bulk production, including genetically stabilized OPV2 strains.

Expected Implications of Globally Coordinated Cessation of Serotype 3 Oral Poliovirus Vaccine (OPV) Before Serotype 1 OPV

“…No prior or post‐OPV cessation modeling explored the risks and benefits of phased cessation of OPV3 within the next 10 years (i.e., 2020–2029). This analysis uses a global model that characterizes the dynamics of population immunity for each poliovirus serotype assuming a path consistent with current GPEI plans and budgeting as the reference case (Kalkowska, Wassilak, Cochi, Pallansch, & Thompson, 2020), which we recently updated to account for epidemiological experiences in late 2019 (Kalkowska, Pallansch et al., 2020) (i.e., reference case 2 or RC2). Briefly, the updated global model (Kalkowska, Wassilak et al., 2020) divides the word into 72 blocks of 10 subpopulations each (approximately 10.7 million people per subpopulation in 2019, Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat, 2019) that mix homogenously in space and heterogeneously by age.…”

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

“…Modeling published in early 2020 suggests that the GPEI remains off track with respect to achieving WPV1 eradication and successful OPV2 cessation [202], although WPV1 eradication remains possible with sufficiently high-intensity OPV vaccination [236,237]. Further modeling can help to quantify the probability of needing to restart OPV2, which a 2020 statistical analysis [238] and modeling study suggest appears likely [239].…”

Poliovirus vaccine options: another step forward

“…The results from these two studies and the Emergency Use Listing represent exciting next steps for a world that already needed more OPV2 before the COVID-19 pandemic. 12,13 This progress will allow for broader use of novel OPV2 and the observation of evidence related to its actual performance in the field. Initial modelling of the potential effects of use of novel OPV2 instead of monovalent OPV2 for outbreak response suggests that novel OPV2 could help to reduce the cases caused by type 2 circulating vaccine-derived poliovirus outbreaks.…”