Comprehensive screening for immunodeficiency-associated vaccine-derived poliovirus: an essential oral poliovirus vaccine cessation risk management strategy

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

BACKGROUND

Polio antiviral drugs (PAVDs) may provide a critical tool in the eradication endgame by stopping poliovirus infections in immunodeficient individuals who may not clear the virus without therapeutic intervention. Although prolonged/chronic poliovirus excreters are rare, they represent a source of poliovirus reintroduction into the general population. Prior studies that assumed the successful cessation of all oral poliovirus vaccine (OPV) use estimated the potential upper bound of the incremental net benefits (INBs) of resource investments in research and development of PAVDs. However, delays in polio eradication, OPV cessation, and the development of PAVDs necessitate an updated economic analysis to reevaluate the costs and benefits of further investments in PAVDs.

METHODS

Using a global integrated model of polio transmission, immunity, vaccine dynamics, risks, and economics, we explore the risks of reintroduction of polio transmission due to immunodeficiency-related vaccine-derived poliovirus (iVDPV) excreters and reevaluate the upper bound of the INBs of PAVDs.

RESULTS

Under the current conditions, for which the use of OPV will likely continue for the foreseeable future, even with successful eradication of type 1 wild poliovirus by the end of 2023 and continued use of Sabin OPV for outbreak response, we estimate an upper bound INB of 60 million US$2019. With >100 million US$2019 already invested in PAVD development and with the introduction of novel OPVs that are less likely to revert to neurovirulence, our analysis suggests the expected INBs of PAVDs would not offset their costs.

CONCLUSIONS

While PAVDs could play an important role in the polio endgame, their expected economic benefits drop with ongoing OPV use and poliovirus transmissions. However, stakeholders may pursue the development of PAVDs as a desired product regardless of their economic benefits.HighlightsWhile polio antiviral drugs could play an important role in the polio endgame, their expected economic benefits continue to drop with delays in polio eradication and the continued use of oral poliovirus vaccines.The incremental net benefits of investments in polio antiviral drug development and screening for immunodeficiency-related circulating polioviruses are small.Limited global resources are better spent on increasing global population immunity to polioviruses to stop and prevent poliovirus transmission.

Health economic analyses of secondary vaccine effects: a systematic review and policy insights

INTRODUCTION

: Numerous analyses demonstrate substantial health economic impacts of primary vaccine effects (preventing or mitigating clinical manifestations of the diseases they target), but vaccines may also be associated with secondary effects, previously known as non-specific, heterologous, or off-target effects.

AREAS COVERED

: We define key concepts to distinguish primary and secondary vaccine effects for health economic analyses, summarized terminology used in different fields, and perform a systematic review of health economic analyses focused on secondary vaccine effects (SVEs).

EXPERT OPINION

: Health economists integrate evidence from multiple fields, which often use incomplete or inconsistent definitions. Like regulators and policy makers, health economists require high-quality evidence of specific effects. Consistent with the limited evidence on mechanisms of action for SVEs, the associated health economic literature remains highly limited, with 4 studies identified by our systematic review. The lack of specific and well-controlled evidence that supports quantification of specific SVEs limits the consideration of these effects in vaccine research, development, regulatory, and recommendation decisions and health economic analyses.

The Molecular Evolution of Type 2 Vaccine-Derived Polioviruses in Individuals with Primary Immunodeficiency Diseases

The oral poliovirus vaccine (OPV), which prevents person-to-person transmission of poliovirus by inducing robust intestinal immunity, has been a crucial tool for global polio eradication. However, polio outbreaks, mainly caused by type 2 circulating vaccine-derived poliovirus (cVDPV2), are increasing worldwide. Meanwhile, immunodeficiency-associated vaccine-derived poliovirus (iVDPV) is considered another risk factor during the final stage of global polio eradication. Patients with primary immunodeficiency diseases are associated with higher risks for long-term iVDPV infections. Although a limited number of chronic iVDPV excretors were reported, the recent identification of a chronic type 2 iVDPV (iVDPV2) excretor in the Philippines highlights the potential risk of inapparent iVDPV infection for expanding cVDPV outbreaks. Further research on the genetic characterizations and molecular evolution of iVDPV2, based on comprehensive iVDPV surveillance, will be critical for elucidating the remaining risk of iVDPV2 during the post-OPV era.

Reflections on Modeling Poliovirus Transmission and the Polio Eradication Endgame

The Global Polio Eradication Initiative (GPEI) partners engaged modelers during the past nearly 20 years to support strategy and policy discussions and decisions, and to provide estimates of the risks, costs, and benefits of different options for managing the polio endgame. Limited efforts to date provided insights related to the validation of the models used for GPEI strategy and policy decisions. However, modeling results only influenced decisions in some cases, with other factors carrying more weight in many key decisions. In addition, the results from multiple modeling groups do not always agree, which supports selection of some strategies and/or policies counter to the recommendations from some modelers but not others. This analysis reflects on our modeling, and summarizes our premises and recommendations, the outcomes of these recommendations, and the implications of key limitations of models with respect to polio endgame strategy. We briefly review the current state of the GPEI given epidemiological experience as of early 2020, which includes failure of the GPEI to deliver on the objectives of its 2013-2018 strategic plan despite full financial support. Looking ahead, we provide context for why the GPEI strategy of global oral poliovirus vaccine (OPV) cessation to end all cases of poliomyelitis looks infeasible given the current state of the GPEI and the failure to successfully stop all transmission of serotype 2 live polioviruses within four years of the April-May 2016 coordinated cessation of serotype 2 OPV use in routine immunization.

Wassilak SG, Cochi SL, Thompson KM. Global Transmission of Live Polioviruses: Updated Dynamic Modeling of the Polio Endgame

Nearly 20 years after the year 2000 target for global wild poliovirus (WPV) eradication, live polioviruses continue to circulate with all three serotypes posing challenges for the polio endgame. We updated a global differential equation-based poliovirus transmission and stochastic risk model to include programmatic and epidemiological experience through January 2020. We used the model to explore the likely dynamics of poliovirus transmission for 2019-2023, which coincides with a new Global Polio Eradication Initiative Strategic Plan. The model stratifies the global population into 72 blocks, each containing 10 subpopulations of approximately 10.7 million people. Exported viruses go into subpopulations within the same block and within groups of blocks that represent large preferentially mixing geographical areas (e.g., continents). We assign representative World Bank income levels to the blocks along with polio immunization and transmission assumptions, which capture some of the heterogeneity across countries while still focusing on global poliovirus transmission dynamics. We also updated estimates of reintroduction risks using available evidence. The updated model characterizes transmission dynamics and resulting polio cases consistent with the evidence through 2019. Based on recent epidemiological experience and prospective immunization assumptions for the 2019-2023 Strategic Plan, the updated model does not show successful eradication of serotype 1 WPV by 2023 or successful cessation of oral poliovirus vaccine serotype 2-related viruses.

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

Globally coordinated cessation of all three serotypes of oral poliovirus vaccine (OPV) represents a critical part of a successful polio endgame, which the Global Polio Eradication Initiative (GPEI) plans to conduct in phases, with serotype 2 OPV cessation completed in mid 2016. Although in 2016 the GPEI expected to globally coordinate cessation of the remaining OPV serotypes (1 and 3) by 2021, continuing transmission of serotype 1 wild polioviruses to date makes those plans obsolete. With increasing time since the last reported polio case caused by serotype 3 wild poliovirus (in November 2012) leading to high confidence about its successful global eradication, the Global Commission for the Certification of Poliomyelitis Eradication recently certified its eradication. Questions now arise about the optimal timing of serotype 3 OPV (OPV3) cessation. Using an integrated global model that characterizes the risks, costs, and benefits of global polio policy and risk management options, we explored the implications of different options for coordinated cessation of OPV3 prior to COVID-19. Globally coordinating cessation of OPV3 as soon as possible offers the opportunity to reduce cases of vaccine-associated paralytic polio globally. In addition, earlier cessation of OPV3 should reduce the risks of creating serotype 3 circulating vaccine-derived polioviruses after OPV3 cessation, which represents a significant threat to the polio endgame given current GPEI plans to reduce preventive OPV supplemental immunization activities starting in 2019.

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

INTRODUCTION

Over the last 20 years (2000-2019) the partners of the Global Polio Eradication Initiative (GPEI) invested in the development and application of mathematical models of poliovirus transmission as well as economics, policy, and risk analyses of polio endgame risk management options, including policies related to poliovirus vaccine use during the polio endgame.

AREAS COVERED

This review provides a historical record of the polio studies published by the three modeling groups that primarily performed the bulk of this work. This review also systematically evaluates the polio transmission and health economic modeling papers published in English in peer-reviewed journals from 2000 to 2019, highlights differences in approaches and methods, shows the geographic coverage of the transmission modeling performed, identified common themes, and discusses instances of similar or conflicting insights or recommendations.

EXPERT OPINION

Polio modeling performed during the last 20 years substantially impacted polio vaccine choices, immunization policies, and the polio eradication pathway. As the polio endgame continues, national preferences for polio vaccine formulations and immunization strategies will likely continue to change. Future modeling will likely provide important insights about their cost-effectiveness and their relative benefits with respect to controlling polio and potentially achieving and maintaining eradication.

Updated modelling of the prevalence of immunodeficiency-associated long-term vaccine-derived poliovirus (iVDPV) excreters

Conditions and evidence continue to evolve related to the prediction of the prevalence of immunodeficiency-associated long-term vaccine-derived poliovirus (iVDPV) excreters, which affect assumptions related to forecasting risks and evaluating potential risk management options. Multiple recent reviews provided information about individual iVDPV excreters, but inconsistencies among the reviews raise some challenges. This analysis revisits the available evidence related to iVDPV excreters and provides updated model estimates that can support future risk management decisions. The results suggest that the prevalence of iVDPV excreters remains highly uncertain and variable, but generally confirms the importance of managing the risks associated with iVDPV excreters throughout the polio endgame in the context of successful cessation of all oral poliovirus vaccine use.

Logistical challenges and assumptions for modeling the failure of global cessation of oral poliovirus vaccine (OPV)

Introduction: The inability to successfully stop all use of oral poliovirus vaccine (OPV) as part of the polio endgame and/or the possibilities of reintroduction of live polioviruses after successful OPV cessation may imply the need to restart OPV production and use, either temporarily or permanently. Areas covered: Complementing prior work that explored the risks of potential OPV restart, we discuss the logistical challenges and implications of restarting OPV in the future, and we develop appropriate assumptions for modeling the possibility of OPV restart. The complexity of phased cessation of the three OPV serotypes implies different potential combinations of OPV use long term. We explore the complexity of polio vaccine choices and key unresolved policy questions that may impact continuing and future use of OPV and/or inactivated poliovirus vaccine (IPV). We then characterize the assumptions required to quantitatively model OPV restart in prospective global-integrated economic policy models for the polio endgame. Expert commentary: Depending on the timing, restarting production of OPV would imply some likely delays associated with ramp-up, re-licensing, and other logistics that would impact the availability and costs of restarting the use of OPV in national immunization programs after globally coordinated cessation of one or more OPV serotypes.

Evaluation of Proactive and Reactive Strategies for Polio Eradication Activities in Pakistan and Afghanistan

Only Pakistan and Afghanistan reported any polio cases caused by serotype 1 wild polioviruses (WPV1s) in 2017. With the dwindling cases in both countries and pressure to finish eradication with the least possible resources, a danger exists of inappropriate prioritization of efforts between the two countries and insufficient investment in the two countries to finish the job. We used an existing differential-equation-based poliovirus transmission and oral poliovirus (OPV) evolution model to simulate a proactive strategy to stop transmission, and different hypothetical reactive strategies that adapt the quality of supplemental immunization activities (SIAs) in response to observed polio cases in Pakistan and Afghanistan. To account for the delay in perception and adaptation, we related the coverage of the SIAs in high-risk, undervaccinated subpopulations to the perceived (i.e., smoothed) polio incidence. Continuation of the current frequency and quality of SIAs remains insufficient to eradicate WPV1 in Pakistan and Afghanistan. Proactive strategies that significantly improve and sustain SIA quality lead to WPV1 eradication and the prevention of circulating vaccine-derived poliovirus (cVDPV) outbreaks. Reactive vaccination efforts that adapt moderately quickly and independently to changes in polio incidence in each country may succeed in WPV1 interruption after several cycles of outbreaks, or may interrupt WPV1 transmission in one country but subsequently import WPV1 from the other country or enable the emergence of cVDPV outbreaks. Reactive vaccination efforts that adapt independently and either more rapidly or more slowly to changes in polio incidence in each country may similarly fail to interrupt WPV1 transmission and result in oscillations of the incidence. Reactive strategies that divert resources to the country of highest priority may lead to alternating large outbreaks. Achieving WPV1 eradication and subsequent successful OPV cessation in Pakistan and Afghanistan requires proactive and sustained efforts to improve vaccination intensity in under-vaccinated subpopulations while maintaining high population immunity elsewhere.

Poliovirus containment risks and their management

AIM

Assess risks related to breaches of poliovirus containment.

METHOD

Using a dynamic transmission model, we explore the variability among different populations in the vulnerability to poliovirus containment breaches as population immunity to transmission declines after oral poliovirus vaccine (OPV) cessation.

RESULTS

Although using OPV instead of wild poliovirus (WPV) seed strains for inactivated poliovirus vaccine (IPV) production offers some expected risk reintroduction of live polioviruses from IPV manufacturing facilities, OPV seed strain releases may become a significant threat within 5-10 years of OPV cessation in areas most conducive to fecal-oral poliovirus transmission, regardless of IPV use.

CONCLUSIONS

Efforts to quantify the risks demonstrate the challenges associated with understanding and managing relatively low-probability and high-consequence containment failure events.

Polio endgame risks and the possibility of restarting the use of oral poliovirus vaccine

INTRODUCTION

Ending all cases of poliomyelitis requires successful cessation of all oral poliovirus vaccine (OPV), but the Global Polio Eradication Initiative (GPEI) partners should consider the possibility of an OPV restart.

AREAS COVERED

We review the risks of continued live poliovirus transmission after OPV cessation and characterize events that led to OPV restart in a global model that focused on identifying optimal strategies for OPV cessation and the polio endgame. Numerous different types of events that occurred since the globally coordinated cessation of serotype 2-containing OPV in 2016 highlight the possibility of continued outbreaks after homotypic OPV cessation. Modeling suggests a high risk of uncontrolled outbreaks once more than around 5,000 homotypic polio cases occur after cessation of an OPV serotype, at which point restarting OPV would become necessary to protect most populations. Current efforts to sunset the GPEI and transition its responsibilities to national governments poses risks that may limit the ability to implement management strategies needed to minimize the probability of an OPV restart.

EXPERT COMMENTARY

OPV restart remains a real possibility, but risk management choices made by the GPEI partners and national governments can reduce the risks of this low-probability but high-consequence event.

Using integrated modeling to support the global eradication of vaccine-preventable diseases

The long-term management of global disease eradication initiatives involves numerous inherently dynamic processes, health and economic trade-offs, significant uncertainty and variability, rare events with big consequences, complex and inter-related decisions, and a requirement for cooperation among a large number of stakeholders. Over the course of more than 16 years of collaborative modeling efforts to support the Global Polio Eradication Initiative, we developed increasingly complex integrated system dynamics models that combined numerous analytical approaches, including differential equation-based modeling, risk and decision analysis, discrete-event and individual-based simulation, probabilistic uncertainty and sensitivity analysis, health economics, and optimization. We discuss the central role of systems thinking and system dynamics in the overall effort and the value of integrating different modeling approaches to appropriately address the trade-offs involved in some of the policy questions. We discuss practical challenges of integrating different analytical tools and we provide our perspective on the future of integrated modeling.

Vaccine-derived poliovirus surveillance in China during 2001-2013: the potential challenge for maintaining polio free status

Background

The goal of polio eradication is to complete elimination and containment of all wild, vaccine-related and Sabin polioviruses. Vaccine-derived poliovirus (VDPV) surveillance in China from 2001–2013 is summarized in this report, which has important implications for the global polio eradication initiative.

Methods

Acute flaccid paralysis (AFP) cases and their contacts with VDPVs isolated from fecal specimens were identified in our AFP surveillance system or by field investigation. Epidemiological and laboratory information for these children were analyzed and the reasons for the VDPV outbreak was explored.

Results

VDPVs were isolated from a total of 49 children in more than two-thirds of Chinese provinces from 2001–2013, including 15 VDPV cases, 15 non-polio AFP cases and 19 contacts of AFP cases or healthy subjects. A total of 3 circulating VDPVs (cVDPVs) outbreaks were reported in China, resulting in 6 cVDPVs cases who had not been vaccinated with oral attenuated poliomyelitis vaccine. Among the 4 immunodeficiency-associated VDPVs (iVDPVs) cases, the longest duration of virus excretion was about 20 months. In addition, one imported VDPV case from Myanmar was detected in Yunnan Province.

Conclusions

Until all wild, vaccine-related and Sabin polioviruses are eradicated in the world, high quality routine immunization and sensitive AFP surveillance should be maintained, focusing efforts on underserved populations in high risk areas.

Prolonged Excretion of Poliovirus among Individuals with Primary Immunodeficiency Disorder: An Analysis of the World Health Organization Registry

Individuals with primary immunodeficiency disorder may excrete poliovirus for extended periods and will constitute the only remaining reservoir of virus after eradication and withdrawal of oral poliovirus vaccine. Here, we analyzed the epidemiology of prolonged and chronic immunodeficiency-related vaccine-derived poliovirus cases in a registry maintained by the World Health Organization, to identify risk factors and determine the length of excretion. Between 1962 and 2016, there were 101 cases, with 94/101 (93%) prolonged excretors and 7/101 (7%) chronic excretors. We documented an increase in incidence in recent decades, with a shift toward middle-income countries, and a predominance of poliovirus type 2 in 73/101 (72%) cases. The median length of excretion was 1.3 years (95% confidence interval: 1.0, 1.4) and 90% of individuals stopped excreting after 3.7 years. Common variable immunodeficiency syndrome and residence in high-income countries were risk factors for long-term excretion. The changing epidemiology of cases, manifested by the greater incidence in recent decades and a shift to from high- to middle-income countries, highlights the expanding risk of poliovirus transmission after oral poliovirus vaccine cessation. To better quantify and reduce this risk, more sensitive surveillance and effective antiviral therapies are needed.

Modeling the costs and benefits of temporary recommendations for poliovirus exporting countries to vaccinate international travelers

Recognizing that infectious agents readily cross international borders, the International Health Regulations Emergency Committee issues Temporary Recommendations (TRs) that include vaccination of travelers from countries affected by public health emergencies, including serotype 1 wild polioviruses (WPV1s). This analysis estimates the costs and benefits of TRs implemented by countries with reported WPV1 during 2014-2016 while accounting for numerous uncertainties. We estimate the TR costs based on programmatic data and prior economic analyses and TR benefits by simulating potential WPV1 outbreaks in the absence of the TRs using the rate and extent of WPV1 importation outbreaks per reported WPV1 case during 2004-2013 and the number of reported WPV1 cases that occurred in countries with active TRs. The benefits of TRs outweigh the costs in 77% of model iterations, resulting in expected incremental net economic benefits of $210 million. Inclusion of indirect costs increases the costs by 13%, the expected savings from prevented outbreaks by 4%, and the expected incremental net benefits by 3%. Despite the considerable costs of implementing TRs, this study provides health and economic justification for these investments in the context of managing a disease in advanced stages of its global eradication.

Lessons From Globally Coordinated Cessation of Serotype 2 Oral Poliovirus Vaccine for the Remaining Serotypes

Background

Comparing model expectations with the experience of oral poliovirus vaccine (OPV) containing serotype 2 (OPV2) cessation can inform risk management for the expected cessation of OPV containing serotypes 1 and 3 (OPV13).

Methods

We compare the expected post-OPV2-cessation OPV2-related viruses from models with the evidence available approximately 6 months after OPV2 cessation. We also model the trade-offs of use vs nonuse of monovalent OPV (mOPV) for outbreak response considering all 3 serotypes.

Results

Although too early to tell definitively, the observed die-out of OPV2-related viruses in populations that attained sufficiently intense trivalent OPV (tOPV) use prior to OPV2 cessation appears consistent with model expectations. As expected, populations that did not intensify tOPV use prior to OPV2 cessation show continued circulation of serotype 2 vaccine-derived polioviruses (VDPVs). Failure to aggressively use mOPV to respond to circulating VDPVs results in a high risk of uncontrolled outbreaks that would require restarting OPV.

Conclusions

Ensuring a successful endgame requires more aggressive OPV cessation risk management than has occurred to date for OPV2 cessation. This includes maintaining high population immunity to transmission up until OPV13 cessation, meeting all prerequisites for OPV cessation, and ensuring sufficient vaccine supply to prevent and respond to outbreaks.

Lessons From the Polio Endgame: Overcoming the Failure to Vaccinate and the Role of Subpopulations in Maintaining Transmission

Background

Recent detections of circulating serotype 2 vaccine-derived poliovirus in northern Nigeria (Borno and Sokoto states) and Pakistan (Balochistan Province) and serotype 1 wild poliovirus in Pakistan, Afghanistan, and Nigeria (Borno) represent public health emergencies that require aggressive response.

Methods

We demonstrate the importance of undervaccinated subpopulations, using an existing dynamic poliovirus transmission and oral poliovirus vaccine evolution model. We review the lessons learned during the polio endgame about the role of subpopulations in sustaining transmission, and we explore the implications of subpopulations for other vaccine-preventable disease eradication efforts.

Results

Relatively isolated subpopulations benefit little from high surrounding population immunity to transmission and will sustain transmission as long as they do not attain high vaccination coverage. Failing to reach such subpopulations with high coverage represents the root cause of polio eradication delays. Achieving and maintaining eradication requires addressing the weakest links, which includes immunizing populations in insecure areas and/or with disrupted or poor-performing health systems and managing the risks of individuals with primary immunodeficiencies who can excrete vaccine-derived poliovirus long-term.

Conclusions

Eradication efforts for vaccine-preventable diseases need to create performance expectations for countries to immunize all people living within their borders and maintain high coverage with appropriate interventions.Keywords. Polio; eradication; transmission; heterogeneity.

Patients with Primary Immunodeficiencies Are a Reservoir of Poliovirus and a Risk to Polio Eradication

Immunodeficiency-associated vaccine-derived polioviruses (iVDPVs) have been isolated from primary immunodeficiency (PID) patients exposed to oral poliovirus vaccine (OPV). Patients may excrete poliovirus strains for months or years; the excreted viruses are frequently highly divergent from the parental OPV and have been shown to be as neurovirulent as wild virus. Thus, these patients represent a potential reservoir for transmission of neurovirulent polioviruses in the post-eradication era. In support of WHO recommendations to better estimate the prevalence of poliovirus excreters among PIDs and characterize genetic evolution of these strains, 635 patients including 570 with primary antibody deficiencies and 65 combined immunodeficiencies were studied from 13 OPV-using countries. Two stool samples were collected over 4 days, tested for enterovirus, and the poliovirus positive samples were sequenced. Thirteen patients (2%) excreted polioviruses, most for less than 2 months following identification of infection. Five (0.8%) were classified as iVDPVs (only in combined immunodeficiencies and mostly poliovirus serotype 2). Non-polio enteroviruses were detected in 30 patients (4.7%). Patients with combined immunodeficiencies had increased risk of delayed poliovirus clearance compared to primary antibody deficiencies. Usually, iVDPV was detected in subjects with combined immunodeficiencies in a short period of time after OPV exposure, most for less than 6 months. Surveillance for poliovirus excretion among PID patients should be reinforced until polio eradication is certified and the use of OPV is stopped. Survival rates among PID patients are improving in lower and middle income countries, and iVDPV excreters are identified more frequently. Antivirals or enhanced immunotherapies presently in development represent the only potential means to manage the treatment of prolonged excreters and the risk they present to the polio endgame.