Risk Analysis
April 2013 Volume 33, Issue 4 Pages 505–749
http://onlinelibrary.wiley.com/doi/10.1111/risa.2013.33.issue-4/issuetoc
Special Issue Theme: Poliovirus Eradication
Introduction
Modeling Poliovirus Risks and the Legacy of Polio Eradication (pages 505–515)
Kimberly M. Thompson
Article first published online: 28 MAR 2013 | DOI: 10.1111/risa.12030
This introduction to the special issue on modeling poliovirus risks provides context about historical efforts to manage polioviruses and reviews the insights from models developed to support risk management and policy development. Following an overview of the contents of the special issue, the introduction explores the road ahead and offers perspective on the legacy of polio eradication.
Original Research Articles
Preeradication Vaccine Policy Options for Poliovirus Infection and Disease Control (pages 516–543)
Kimberly M. Thompson, Mark A. Pallansch, Radboud J. Duintjer Tebbens, Steve G. Wassilak, Jong-Hoon Kim and Stephen L. Cochi
Article first published online: 5 MAR 2013 | DOI: 10.1111/risa.12019
Abstract
With the circulation of wild poliovirus (WPV) types 1 and 3 continuing more than a decade after the original goal of eradicating all three types of WPVs by 2000, policymakers consider many immunization options as they strive to stop transmission in the remaining endemic and outbreak areas and prevent reintroductions of live polioviruses into nonendemic areas. While polio vaccination choices may appear simple, our analysis of current options shows remarkable complexity. We offer important context for current and future polio vaccine decisions and policy analyses by developing decision trees that clearly identify potential options currently used by countries as they evaluate national polio vaccine choices. Based on a comprehensive review of the literature we (1) identify the current vaccination options that national health leaders consider for polio vaccination, (2) characterize current practices and factors that appear to influence national and international choices, and (3) assess the evidence of vaccine effectiveness considering sources of variability between countries and uncertainties associated with limitations of the data. With low numbers of cases occurring globally, the management of polio risks might seem like a relatively low priority, but stopping live poliovirus circulation requires making proactive and intentional choices to manage population immunity in the remaining endemic areas and to prevent reestablishment in nonendemic areas. Our analysis shows remarkable variability in the current national polio vaccine product choices and schedules, with combination vaccine options containing inactivated poliovirus vaccine and different formulations of oral poliovirus vaccine making choices increasingly difficult for national health leaders.
Expert Review on Poliovirus Immunity and Transmission (pages 544–605)
Radboud J. Duintjer Tebbens, Mark A. Pallansch, Konstantin M. Chumakov, Neal A. Halsey, Tapani Hovi, Philip D. Minor, John F. Modlin, Peter A. Patriarca, Roland W. Sutter, Peter F. Wright, Steven G. F. Wassilak, Stephen L. Cochi, Jong-Hoon Kim and Kimberly M. Thompson
Article first published online: 15 JUL 2012 | DOI: 10.1111/j.1539-6924.2012.01864.x
Abstract
Successfully managing risks to achieve wild polioviruses (WPVs) eradication and address the complexities of oral poliovirus vaccine (OPV) cessation to stop all cases of paralytic poliomyelitis depends strongly on our collective understanding of poliovirus immunity and transmission. With increased shifting from OPV to inactivated poliovirus vaccine (IPV), numerous risk management choices motivate the need to understand the tradeoffs and uncertainties and to develop models to help inform decisions. The U.S. Centers for Disease Control and Prevention hosted a meeting of international experts in April 2010 to review the available literature relevant to poliovirus immunity and transmission. This expert review evaluates 66 OPV challenge studies and other evidence to support the development of quantitative models of poliovirus transmission and potential outbreaks. This review focuses on characterization of immunity as a function of exposure history in terms of susceptibility to excretion, duration of excretion, and concentration of excreted virus. We also discuss the evidence of waning of host immunity to poliovirus transmission, the relationship between the concentration of poliovirus excreted and infectiousness, the importance of different transmission routes, and the differences in transmissibility between OPV and WPV. We discuss the limitations of the available evidence for use in polio risk models, and conclude that despite the relatively large number of studies on immunity, very limited data exist to directly support quantification of model inputs related to transmission. Given the limitations in the evidence, we identify the need for expert input to derive quantitative model inputs from the existing data.
Review and Assessment of Poliovirus Immunity and Transmission: Synthesis of Knowledge Gaps and Identification of Research Needs (pages 606–646)
Radboud J. Duintjer Tebbens, Mark A. Pallansch, Konstantin M. Chumakov, Neal A. Halsey, Tapani Hovi, Philip D. Minor, John F. Modlin, Peter A. Patriarca, Roland W. Sutter, Peter F. Wright, Steven G.F. Wassilak, Stephen L. Cochi, Jong-Hoon Kim and Kimberly M. Thompson
Article first published online: 28 MAR 2013 | DOI: 10.1111/risa.12031
Abstract
With the intensifying global efforts to eradicate wild polioviruses, policymakers face complex decisions related to achieving eradication and managing post eradication risks. These decisions and the expanding use of inactivated poliovirus vaccine (IPV) trigger renewed interest in poliovirus immunity, particularly the role of mucosal immunity in the transmission of polioviruses. Sustained high population immunity to poliovirus transmission represents a key prerequisite to eradication, but poliovirus immunity and transmission remain poorly understood despite decades of studies. In April 2010, the U.S. Centers for Disease Control and Prevention convened an international group of experts on poliovirus immunology and virology to review the literature relevant for modeling poliovirus transmission, develop a consensus about related uncertainties, and identify research needs. This article synthesizes the quantitative assessments and research needs identified during the process. Limitations in the evidence from oral poliovirus vaccine (OPV) challenge studies and other relevant data led to differences in expert assessments, indicating the need for additional data, particularly in several priority areas for research: (1) the ability of IPV-induced immunity to prevent or reduce excretion and affect transmission, (2) the impact of waning immunity on the probability and extent of poliovirus excretion, (3) the relationship between the concentration of poliovirus excreted and infectiousness to others in different settings, and (4) the relative role of fecal-oral versus oropharyngeal transmission. This assessment of current knowledge supports the immediate conduct of additional studies to address the gaps.
Modeling Population Immunity to Support Efforts to End the Transmission of Live Polioviruses (pages 647–663)
Kimberly M. Thompson, Mark A. Pallansch, Radboud J. Duintjer Tebbens, Steve G. Wassilak and Stephen L. Cochi
Article first published online: 17 SEP 2012 | DOI: 10.1111/j.1539-6924.2012.01891.x
Abstract
Eradication of wild poliovirus (WPV) types 1 and 3, prevention and cessation of circulating vaccine-derived polioviruses, and achievement and maintenance of a world free of paralytic polio cases requires active risk management by focusing on population immunity and coordinated cessation of oral poliovirus vaccine (OPV). We suggest the need for a complementary and different conceptual approach to achieve eradication compared to the current case-based approach using surveillance for acute flaccid paralysis (AFP) to identify symptomatic poliovirus infections. Specifically, we describe a modeling approach to characterize overall population immunity to poliovirus transmission. The approach deals with the realities that exposure to live polioviruses (e.g., WPV, OPV) and/or vaccination with inactivated poliovirus vaccine provides protection from paralytic polio (i.e., disease), but does not eliminate the potential for reinfection or asymptomatic participation in poliovirus transmission, which may increase with time because of waning immunity. The AFP surveillance system provides evidence of symptomatic poliovirus infections detected, which indicate immunity gaps after outbreaks occur, and this system represents an appropriate focus for controlling disease outbreaks. We describe a conceptual dynamic model to characterize population immunity to poliovirus transmission that helps identify risks created by immunity gaps before outbreaks occur, which provides an opportunity for national and global policymakers to manage the risk of poliovirus and prevent outbreaks before they occur. We suggest that dynamically modeling risk represents an essential tool as the number of cases approaches zero.
World Health Organization Regional Assessments of the Risks of Poliovirus Outbreaks (pages 664–679)
Sara A. Lowther, Sigrun Roesel, Patrick O’Connor, Mauricio Landaverde, George Oblapenko, Sergei Deshevoi, Goel Ajay, Ann Buff, Hala Safwat, Mbaye Salla, Rudi Tangermann, Nino Khetsuriani, Rebecca Martin and Steven Wassilak
Article first published online: 23 MAR 2013 | DOI: 10.1111/risa.12032
Abstract
While global polio eradication requires tremendous efforts in countries where wild polioviruses (WPVs) circulate, numerous outbreaks have occurred following WPV importation into previously polio-free countries. Countries that have interrupted endemic WPV transmission should continue to conduct routine risk assessments and implement mitigation activities to maintain their polio-free status as long as wild poliovirus circulates anywhere in the world. This article reviews the methods used by World Health Organization (WHO) regional offices to qualitatively assess risk of WPV outbreaks following an importation. We describe the strengths and weaknesses of various risk assessment approaches, and opportunities to harmonize approaches. These qualitative assessments broadly categorize risk as high, medium, or low using available national information related to susceptibility, the ability to rapidly detect WPV, and other population or program factors that influence transmission, which the regions characterize using polio vaccination coverage, surveillance data, and other indicators (e.g., sanitation), respectively. Data quality and adequacy represent a challenge in all regions. WHO regions differ with respect to the methods, processes, cut-off values, and weighting used, which limits comparisons of risk assessment results among regions. Ongoing evaluation of indicators within regions and further harmonization of methods between regions are needed to effectively plan risk mitigation activities in a setting of finite resources for funding and continued WPV circulation.
Oral Poliovirus Vaccine Evolution and Insights Relevant to Modeling the Risks of Circulating Vaccine-Derived Polioviruses (cVDPVs) (pages 680–702)
Radboud J. Duintjer Tebbens, Mark A. Pallansch, Jong-Hoon Kim, Cara C. Burns, Olen M. Kew, M. Steven Oberste, Ousmane M. Diop, Steven G.F. Wassilak, Stephen L. Cochi and Kimberly M. Thompson
Article first published online: 7 MAR 2013 | DOI: 10.1111/risa.12022
Abstract
The live, attenuated oral poliovirus vaccine (OPV) provides a powerful tool for controlling and stopping the transmission of wild polioviruses (WPVs), although the risks of vaccine-associated paralytic polio (VAPP) and circulating vaccine-derived poliovirus (cVDPV) outbreaks exist as long as OPV remains in use. Understanding the dynamics of cVDPV emergence and outbreaks as a function of population immunity and other risk factors may help to improve risk management and the development of strategies to respond to possible outbreaks. We performed a comprehensive review of the literature related to the process of OPV evolution and information available from actual experiences with cVDPV outbreaks. Only a relatively small fraction of poliovirus infections cause symptoms, which makes direct observation of the trajectory of OPV evolution within a population impractical and leads to significant uncertainty. Despite a large global surveillance system, the existing genetic sequence data largely provide information about transmitted virulent polioviruses that caused acute flaccid paralysis, and essentially no data track the changes that occur in OPV sequences as the viruses transmit largely asymptomatically through real populations with suboptimal immunity. We updated estimates of cVDPV risks based on actual experiences and identified the many limitations in the existing data on poliovirus transmission and immunity and OPV virus evolution that complicate modeling. Modelers should explore the space of potential model formulations and inputs consistent with the available evidence and future studies should seek to improve our understanding of the OPV virus evolution process to provide better information for policymakers working to manage cVDPV risks.
Characterizing Poliovirus Transmission and Evolution: Insights from Modeling Experiences with Wild and Vaccine-Related Polioviruses (pages 703–749)
Radboud J. Duintjer Tebbens, Mark A. Pallansch, Dominika A. Kalkowska, Steven G. F. Wassilak, Stephen L. Cochi and Kimberly M. Thompson
Article first published online: 22 MAR 2013 | DOI: 10.1111/risa.12044
Abstract
With national and global health policymakers facing numerous complex decisions related to achieving and maintaining polio eradication, we expanded our previously developed dynamic poliovirus transmission model using information from an expert literature review process and including additional immunity states and the evolution of oral poliovirus vaccine (OPV). The model explicitly considers serotype differences and distinguishes fecal-oral and oropharyngeal transmission. We evaluated the model by simulating diverse historical experiences with polioviruses, including one country that eliminated wild poliovirus using both OPV and inactivated poliovirus vaccine (IPV) (USA), three importation outbreaks of wild poliovirus (Albania, the Netherlands, Tajikistan), one situation in which no circulating vaccine-derived polioviruses (cVDPVs) emerge despite annual OPV use and cessation (Cuba), three cVDPV outbreaks (Haiti, Madura Island in Indonesia, northern Nigeria), one area of current endemic circulation of all three serotypes (northern Nigeria), and one area with recent endemic circulation and subsequent elimination of multiple serotypes (northern India). We find that when sufficient information about the conditions exists, the model can reproduce the general behavior of poliovirus transmission and outbreaks while maintaining consistency in the generic model inputs. The assumption of spatially homogeneous mixing remains a significant limitation that affects the performance of the differential equation-based model when significant heterogeneities in immunity and mixing may exist. Further studies on OPV virus evolution and improved understanding of the mechanisms of mixing and transmission may help to better characterize poliovirus transmission in populations. Broad application of the model promises to offer insights in the context of global and national policy and economic models.