The impact of disruptions caused by the COVID-19 pandemic on global polio eradication.

In early 2020, the COVID-19 pandemic led to substantial disruptions in global activities. The disruptions also included intentional and unintentional reductions in health services, including immunization campaigns against the transmission of wild poliovirus (WPV) and persistent serotype 2 circulating vaccine-derived poliovirus (cVDPV2). Building on a recently updated global poliovirus transmission and Sabin-strain oral poliovirus vaccine (OPV) evolution model, we explored the implications of immunization disruption and restrictions of human interactions (i.e., population mixing) on the expected incidence of polio and on the resulting challenges faced by the Global Polio Eradication Initiative (GPEI). We demonstrate that with some resumption of activities in the fall of 2020 to respond to cVDPV2 outbreaks and full resumption on January 1, 2021 of all polio immunization activities to pre-COVID-19 levels, the GPEI could largely mitigate the impact of COVID-19 to the delays incurred. The relative importance of reduced mixing (leading to potentially decreased incidence) and reduced immunization (leading to potentially increased expected incidence) depends on the timing of the effects. Following resumption of immunization activities, the GPEI will likely face similar barriers to eradication of WPV and elimination of cVDPV2 as before COVID-19. The disruptions from the COVID-19 pandemic may further delay polio eradication due to indirect effects on vaccine and financial resources.

Serotype 2 oral poliovirus vaccine (OPV2) choices and the consequences of delaying outbreak response.

The Global Polio Eradication Initiative (GPEI) faces substantial challenges with managing outbreaks of serotype 2 circulating vaccine-derived polioviruses (cVDPV2s) in 2021. A full five years after the globally coordinated removal of serotype 2 oral poliovirus vaccine (OPV2) from trivalent oral poliovirus vaccine (tOPV) for use in national immunization programs, cVDPV2s did not die out. Since OPV2 cessation, responses to outbreaks caused by cVDPV2s mainly used serotype 2 monovalent OPV (mOPV2) from a stockpile. A novel vaccine developed from a genetically stabilized OPV2 strain (nOPV2) promises to potentially facilitate outbreak response with lower prospective risks, although its availability and properties in the field remain uncertain. Using an established global poliovirus transmission model and building on a related analysis that characterized the impacts of disruptions in GPEI activities caused by the COVID-19 pandemic, we explore the implications of trade-offs associated with delaying outbreak response to avoid using mOPV2 by waiting for nOPV2 availability (or equivalently, delayed responses waiting for national validation of meeting the criteria for nOPV2 initial use). Consistent with prior modeling, responding as quickly as possible with available mOPV2 promises to reduce the expected burden of disease in the outbreak population and to reduce the chances for the outbreak virus to spread to other areas. Delaying cVDPV2 outbreak response (e.g., modeled as no response January-June 2021) to wait for nOPV2 can considerably increase the total expected cases (e.g., by as many as 1,300 cVDPV2 cases in the African region during 2021-2023) and increases the likelihood of triggering the need to restart widescale preventive use of an OPV2-containing vaccine in national immunization programs that use OPV. Countries should respond to any cVDPV2 outbreaks quickly with rounds that achieve high coverage using any available OPV2, and plan to use nOPV2, if needed, once it becomes widely available based on evidence that it is as effective but safer in populations than mOPV2.

Update on Vaccine-Derived Poliovirus Outbreaks - Worldwide, January 2021-December 2022.

Circulating vaccine-derived poliovirus (cVDPV) outbreaks* can occur when oral poliovirus vaccine (OPV, containing one or more Sabin-strain serotypes 1, 2, and 3) strains undergo prolonged circulation in under-vaccinated populations, resulting in genetically reverted neurovirulent virus (1,2). Following declaration of the eradication of wild poliovirus type 2 in 2015 and the global synchronized switch from trivalent OPV (tOPV, containing Sabin-strain types 1, 2, and 3) to bivalent OPV (bOPV, containing types 1 and 3 only) for routine immunization activities† in April 2016 (3), cVDPV type 2 (cVDPV2) outbreaks have been reported worldwide (4). During 2016-2020, immunization responses to cVDPV2 outbreaks required use of Sabin-strain monovalent OPV2, but new VDPV2 emergences could occur if campaigns did not reach a sufficiently high proportion of children. Novel oral poliovirus vaccine type 2 (nOPV2), a more genetically stable vaccine than Sabin OPV2, was developed to address the risk for reversion to neurovirulence and became available in 2021. Because of the predominant use of nOPV2 during the reporting period, supply replenishment has frequently been insufficient for prompt response campaigns (5). This report describes global cVDPV outbreaks during January 2021-December 2022 (as of February 14, 2023) and updates previous reports (4). During 2021-2022, there were 88 active cVDPV outbreaks, including 76 (86%) caused by cVDPV2. cVDPV outbreaks affected 46 countries, 17 (37%) of which reported their first post-switch cVDPV2 outbreak. The total number of paralytic cVDPV cases during 2020-2022 decreased by 36%, from 1,117 to 715; however, the proportion of all cVDPV cases that were caused by cVDPV type 1 (cVDPV1) increased from 3% in 2020 to 18% in 2022, including the occurrence of cocirculating cVDPV1 and cVDPV2 outbreaks in two countries. The increased proportion of cVDPV1 cases follows a substantial decrease in global routine immunization coverage and suspension of preventive immunization campaigns during the COVID-19 pandemic (2020-2022) (6); outbreak responses in some countries were also suboptimal. Improving routine immunization coverage, strengthening poliovirus surveillance, and conducting timely and high-quality supplementary immunization activities (SIAs) in response to cVDPV outbreaks are needed to interrupt cVDPV transmission and reach the goal of no cVDPV isolations in 2024.

Progress Toward Polio Eradication - Worldwide, January 2019-June 2021.

In 1988, when the Global Polio Eradication Initiative (GPEI) began, polio paralyzed >350,000 children across 125 countries. Today, only one of three wild poliovirus serotypes, type 1 (WPV1), remains in circulation in only two countries, Afghanistan and Pakistan. This report summarizes progress toward global polio eradication during January 1, 2019-June 30, 2021 and updates previous reports (1,2). In 2020, 140 cases of WPV1 were reported, including 56 in Afghanistan (a 93% increase from 29 cases in 2019) and 84 in Pakistan (a 43% decrease from 147 cases in 2019). As GPEI focuses on the last endemic WPV reservoirs, poliomyelitis outbreaks caused by circulating vaccine-derived poliovirus (cVDPV) have emerged as a result of attenuated oral poliovirus vaccine (OPV) virus regaining neurovirulence after prolonged circulation in underimmunized populations (3). In 2020, 32 countries reported cVDPV outbreaks (four type 1 [cVDPV1], 26 type 2 [cVDPV2] and two with outbreaks of both); 13 of these countries reported new outbreaks. The updated GPEI Polio Eradication Strategy 2022-2026 (4) includes expanded use of the type 2 novel oral poliovirus vaccine (nOPV2) to avoid new emergences of cVDPV2 during outbreak responses (3). The new strategy deploys other tactics, such as increased national accountability, and focused investments for overcoming the remaining barriers to eradication, including program disruptions and setbacks caused by the COVID-19 pandemic.

Updated Characterization of Outbreak Response Strategies for 2019-2029: Impacts of Using a Novel Type 2 Oral Poliovirus Vaccine Strain.

Delays in achieving the global eradication of wild poliovirus transmission continue to postpone subsequent cessation of all oral poliovirus vaccine (OPV) use. Countries must stop OPV use to end all cases of poliomyelitis, including vaccine-associated paralytic polio (VAPP) and cases caused by vaccine-derived polioviruses (VDPVs). The Global Polio Eradication Initiative (GPEI) coordinated global cessation of all type 2 OPV (OPV2) use in routine immunization in 2016 but did not successfully end the transmission of type 2 VDPVs (VDPV2s), and consequently continues to use type 2 OPV (OPV2) for outbreak response activities. Using an updated global poliovirus transmission and OPV evolution model, we characterize outbreak response options for 2019-2029 related to responding to VDPV2 outbreaks with a genetically stabilized novel OPV (nOPV2) strain or with the currently licensed monovalent OPV2 (mOPV2). Given uncertainties about the properties of nOPV2, we model different assumptions that appear consistent with the evidence on nOPV2 to date. Using nOPV2 to respond to detected cases may reduce the expected VDPV and VAPP cases and the risk of needing to restart OPV2 use in routine immunization compared to mOPV2 use for outbreak response. The actual properties, availability, and use of nOPV2 will determine its effects on type 2 poliovirus transmission in populations. Even with optimal nOPV2 performance, countries and the GPEI would still likely need to restart OPV2 use in routine immunization in OPV-using countries if operational improvements in outbreak response to stop the transmission of cVDPV2s are not implemented effectively.

Progress Toward Poliovirus Containment Implementation - Worldwide, 2019-2020.

Since 1988, when World Health Organization (WHO) Member States and partners launched the Global Polio Eradication Initiative, the number of wild poliovirus (WPV) cases has declined from 350,000 in 125 countries to 176 in only two countries in 2019 (1). The Global Commission for the Certification of Poliomyelitis Eradication (GCC) declared two of the three WPV types, type 2 (WPV2) and type 3 (WPV3), eradicated globally in 2015 and 2019, respectively (1). Wild poliovirus type 1 (WPV1) remains endemic in Afghanistan and Pakistan (1). Containment under strict biorisk management measures is vital to prevent reintroduction of eradicated polioviruses into communities from poliovirus facilities. In 2015, Member States committed to contain type 2 polioviruses (PV2) in poliovirus-essential facilities (PEFs) certified in accordance with a global standard (2). Member states agreed to report national PV2 inventories annually, destroy unneeded PV2 materials, and, if retaining PV2 materials, establish national authorities for containment (NACs) and a PEF auditing process. Since declaration of WPV3 eradication in October 2019, these activities are also required with WPV3 materials. Despite challenges faced during 2019-2020, including the coronavirus disease 2019 (COVID-19) pandemic, the global poliovirus containment program continues to work toward important milestones. To maintain progress, all WHO Member States are urged to adhere to the agreed containment resolutions, including officially establishing legally empowered NACs and submission of PEF Certificates of Participation.

Progress Toward Polio Eradication - Worldwide, January 2018-March 2020.

Since the Global Polio Eradication Initiative (GPEI) was established in 1988, two of the three wild poliovirus (WPV) serotypes (types 2 and 3) have been eradicated.* Transmission of WPV type 1 (WPV1) remains uninterrupted only in Afghanistan and Pakistan. This report summarizes progress toward global polio eradication during January 1, 2018-March 31, 2020 and updates previous reports (1,2). In 2019, Afghanistan and Pakistan reported the highest number of WPV1 cases (176) since 2014. During January 1-March 31, 2020 (as of June 19), 54 WPV1 cases were reported, an approximate fourfold increase from 12 cases during the corresponding period in 2019. Paralytic poliomyelitis can also be caused by circulating vaccine-derived poliovirus (cVDPV), which emerges when attenuated oral poliovirus vaccine (OPV) virus reverts to neurovirulence following prolonged circulation in underimmunized populations (3). Since the global withdrawal of type 2-containing OPV (OPV2) in April 2016, cVDPV type 2 (cVDPV2) outbreaks have increased in number and geographic extent (4). During January 2018-March 2020, 21 countries reported 547 cVDPV2 cases. Complicating increased poliovirus transmission during 2020, the coronavirus disease 2019 (COVID-19) pandemic and mitigation efforts have resulted in suspension of immunization activities and disruptions to poliovirus surveillance. When the COVID-19 emergency subsides, enhanced support will be needed to resume polio eradication field activities.