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Working Papers Group 1: Potential Economic Benefits Of WHO Strategic Advisory Group on Malaria Eradication Working papers Group 1: Potential economic benefits of malaria elimination and eradication Title The malaria elimination game Author Scott Barrett (Columbia University) Contact [email protected] 1 This working paper was commissioned by the WHO Strategic Advisory Group on Malaria Eradication between 2016 and 2019. The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Health Organization concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. The mention of specific companies or of certain manufacturers’ products does not imply that they are endorsed or recommended by the World Health Organization in preference to others of a similar nature that are not mentioned. Errors and omissions excepted, the names of proprietary products are distinguished by initial capital letters. This report contains the collective views of the authors and does not necessarily represent the decisions or the stated policies of the World Health Organization or the Strategic Advisory Group on Malaria Eradication. 2 The Malaria Elimination Game Scott Barrett Columbia University Preliminary draft 18 November 2018 Abstract. Sometime in the future, possibly within a few years, the nations of the world will meet in Geneva to decide whether or not to declare a goal to eradicate malaria, a disease that currently puts about half of the world’s population at risk, that causes over 200 million cases of infection a year, and that kills over 400,000 people annually, mainly young children in sub-Saharan Africa. This paper, still preliminary, develops a model to investigate the incentives that individuals have to protect themselves from malaria, and that nation states have to control and eliminate malaria unilaterally. (In the future I can incorporate an analysis of the incentives all countries together have to eradicate malaria worldwide.) The paper builds on the classic Ross-Macdonald epidemiology model of the dynamics of transmission within and between mosquito and human host populations, and allows the players (individuals at the local/national level; in subsequent work, this could be extended to nation states at the regional/global level) to choose their control actions. As malaria control abounds with externalities, and as the conditions that shape behavior vary widely around the world, this model should be helpful in identifying the challenges that must be overcome in pursuit of this audacious goal. 1. Introduction When the Global Malaria Eradication Program was launched in 1955, its success was far from certain. Critics thought that elimination would be difficult in areas with poor health systems. They also expressed concern that, should eradication fail, and efforts to eliminate malaria be scaled back, individuals who had lost immunity in the interim period, thanks to the program’s success, would be at risk from a resurgence (Nájera, González-Silva, and Alonso 2011: 2). Despite these warnings, the program succeeded spectacularly in some places in the early years. In Sri Lanka, for example, incidence fell from around one million cases a year to just 17 cases in 1963. However, just as it seemed that malaria was about to be eliminated in Sri Lanka, the case count there started to increase, and by 1969 there were once again over half a million cases on the island. It was also at this time that the entire global effort was suspended, with the operational goal switching from eradication to control. For decades after this, transmission of malaria in Sri Lanka and elsewhere remained high, with malaria deaths nearly doubling between 1980 and 2004 (Murray et al. 2012). Starting around 2000, however, the situation started to reverse, and malaria 1 seemed once again in retreat in the face of determined public action.1 The gains were greatest in Africa, a continent that had been neglected by the previous eradication campaign. By 2015, prevalence of P. falciparum, the deadliest form of malaria, had been cut in half in Africa (Bhatt et al. 2015). Elsewhere, the map of malaria endemic countries was shrinking (Feachem et al. 2010). Against the background of these positive trends, Bill and Melinda Gates declared in 2007 that the world should recommit itself to the goal of eradicating malaria. Some years from now, the World Health Assembly will consider their challenge. Should the world try again to eradicate malaria? In his speech calling for malaria eradication, Bill Gates said, “We should declare the goal of eradicating malaria because we can eradicate malaria.”2 In truth, however, we’ll never know if we can eradicate malaria unless and until we do eradicate malaria. Polio has been on the cusp of eradication for more than a decade, and still it isn’t 100% certain that polio can be eradicated. The feasibility of eradication is a prerequisite for pursuing the goal of eradication; it shouldn’t be the reason for pursuing this goal. The reason for pursuing eradication should be the belief that, by doing so, the world would be better off. This places the question posed at the start of this paragraph in the realm of cost-benefit analysis. A number of key features of eradication must be reflected in a cost-benefit analysis of the decision to eradicate. First, the reason for eradicating a disease isn’t only to eliminate infections; it is also—perhaps even mainly—to avoid the need to control the disease in the future. Hence, a key component of a cost-benefit analysis is identification of the alternative to eradication: the level of “optimal control,” and this needn’t be the current level of control. Second, eradication may prove unattainable for many reasons. It’s impossible to know how a complex system involving humans, vectors, and pathogens will play out as it is pushed into completely uncharted territory. A benefit-cost analysis of eradication needs to incorporate the probability that eradication will succeed. Third, analysis must also address the post-eradication risks. For example, if the four species of parasite that currently infect humans are eradicated, might another species evolve to fill the opened niche? What actions are needed to guard against the risk of re-emergence? Finally, many of the tools deployed to control malaria are prone to resistance, and the risks of resistant strains emerging and spreading increase as control methods are stepped up, as is needed if the goal is to eradicate. The risk of failure may thus not be that the world will revert to the level of control that is optimal today. Resistance fueled by an attempt to eradicate malaria may mean that this level of control is no longer attainable. An allied set of issues concerns the incentives individuals in malaria endemic countries have to stamp out transmission, even when the risk to them of being infected is very low; the incentives endemic countries have to pursue elimination 1 Snow et al. (2017) find that the cycles and trends in malaria prevalence over the last 115 years can’t be explained by deliberate intervention or underlying trends like climate change. 2 https://www.gatesfoundation.org/media-center/speeches/2007/10/bill-gates-malaria-forum. 2 within their jurisdictions, even as malaria continues to enter the country from outside; and the incentives all countries together have to see the enterprise through to completion. This paper is a preliminary inquiry into these issues. I am unable to address all of these matters here. In particular, I ignore the problem of resistance. I also ignore the spatial aspects of control/elimination possibly leading to eradication. (If funding were available, I would be interested in taking up these subjects and perhaps others in future work.) There is an enormous literature on the mathematics of malaria transmission. The novelty in my analysis is that it incorporates human behavior. This is especially important because malaria control is shot through with externalities—situations in which the actions of each player affects others (positively or negatively), and no player has an incentive to take these spillovers into account. Because interactions among players are important, malaria control, elimination, and eradication are best analyzed using the tools of game theory. Previously, I have used these tools to explore eradication of vaccine-preventable diseases (Barrett 2003, 2006, 2010, and 2013; Barrett and Hoel 2007). Malaria, however, is much more complex—and, as we shall see, much more interesting. 2. Comparison with other diseases Though the earlier effort to eradicate malaria failed, the campaign to eradicate smallpox succeeded spectacularly (smallpox eradication being certified in 1979), and the latter success has had a more enduring and bewitching effect on global public health policy discussions than the former failure. If we succeeded with smallpox, why not eradicate other diseases? The leaders of the smallpox campaign seem to have anticipated this reaction, for in the final chapter of their comprehensive account of that effort, they offer this warning: “Disease eradication is unquestionably an attractive goal, but the difficulty of its achievement should not be underestimated. It must be borne in mind that when the smallpox eradication programme began, the prospects for its success were more favourable than for any other disease eradication programme that might be envisaged today. The technical feasibility of interrupting the transmission of the smallpox virus had already been demonstrated both in the industrialized countries and in many developing ones; an inexpensive, highly effective vaccine was available; and there was a substantial political commitment to the achievement of the goal that had been set. Nevertheless, however favorable the circumstances, success was by no means a certainty even during the concluding years or even the final months of the programme.
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