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Article Excerpt
1. Introduction
Economic evaluations represent well-accepted tools to help inform decision makers in public health. For example, standardized cost-effectiveness analyses for different public health interventions facilitate allocation of available resources toward those interventions that yield the highest expected societal utility per unit of monetary investment (Gold et al. 1996). However, economic evaluations typically provide a relatively static representation of the situation. In the context of infectious diseases, Edmunds et al. (1999) called for a more dynamic perspective and argued for the use of mathematical infection transmission models (Anderson and May 1991) in cost-effectiveness analyses (Edmunds et al. 1999). Researchers subsequently demonstrated the utility of these models in cost-effectiveness analyses for actual diseases, including varicella (Brisson and Edmunds 2003) and polio (Thompson and Duintjer Tebbens 2006). Furthermore, a growing body of work based on optimal control theory exists that explores the optimality of vaccination strategies in dynamic infection transmission models (see, for example, Greenhalgh 1988; Hadeler and Muller 1993; Muller 1998, 2000).
Eradicable diseases differ from other diseases with respect to the dynamics and economics of vaccination decisions. Geoffard and Philipson (1997) demonstrated the challenge of eradicating a disease if prevalence drives the demand for vaccines. Barrett and colleagues (Barrett 2004, Barrett and Hoel 2007) showed that when a vaccine-preventable disease is eradicable and offers financial dividends associated with the cessation of vaccination into the future, a policy of high control but no eradication is never economically optimal.
Figure 1 summarizes the history of several diseases targeted for global eradication since the establishment of the World Health Organization (WHO). Smallpox remains the only disease successfully eradicated to date, with the last naturally occurring case in 1977 (Fenner et al. 1988). Despite the estimated significant benefit: cost return for the investment by developed countries in the intensive smallpox eradication program that completed eradication in developing countries, a persistent lack of adequate funding jeopardized the eradication effort (Fenner et al. 1988, Barrett 2006).
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Global wild poliovirus and dracunculiasis (Guinea worm) eradication efforts started in the late 1980s and resulted in very impressive reductions in incidence (i.e., new disease cases occurring per unit of time). Nevertheless, political, operational, and financial challenges plagued both eradication programs and resulted in setbacks in their progress. Watts (1998, p. 808) discussed "Ministry of Health officials thinking that they could take the eradication goal of December 1995 as an accomplished fact andquestioning the need to continue using scarce funds for dracunculiasis surveillance." Hopkins et al. (2005, p. 674) reported a "lack of urgency in responding to suspected cases ... [and] inadequacy of surveillance for dracunculiasis in ... [areas] that have reduced or apparently eliminated the disease at great cost." Similarly, the substantial financial commitment required to finish polio eradication has led some to suggest that "the time has come for the global strategy for polio to be shifted from 'eradication' to 'effective control,'" which they suggest would "benefit the fight against the many vaccine preventable diseases" (Arita et al. 2006, p. 853). However, assuming that wild poliovirus eradication is technically achievable, our recent analysis shows that abandoning eradication in favor of control out of a concern of high costs to eliminate the few remaining cases will lead to higher long-term economic and public health costs (Thompson and Duintjer Tebbens 2007).
Adopted as a goal by the World Health Assembly in 1955, the WHO abandoned global malaria eradication in 1969 after investing over $1 billion in external funding (converted to 2006 net present value from data in Fenner et al. 1988, p. 384). In hindsight, due to vector and parasite resistance and other factors, the feasibility of malaria eradication with the strategy in use at the time remains questionable (Centers for Disease Control and Prevention 1993, Carter and Mendis 2002). In 1998, the WHO launched the Roll Back Malaria Initiative to halt the resurgence of malaria in many developing countries. Although the geographical extent of malaria endemicity in tropical Africa has remained unchanged (Carter and Mendis 2002), other countries have experienced large changes. For example, Figure 1(b) shows the experience with malaria in the past six decades in India, where incidence decreased from an estimated 75 million cases in 1947 to less than 50,000 cases in 1961. Following shortages of DDT and then later technical, financial, and operational problems, the incidence of malaria in India resurged to almost 6.5 million cases in 1976, triggering modified control policies. In 1972, Scholtens, while acknowledging operational hurdles and problems with mosquito resistance in India, wrote that "the problem is one of near-success in an environment with an excess of problems clamoring for attention," and "as malaria recedes to a low level other pressing health and social problems exert irresistible demands for available resources" (Scholtens et al. 1972, p. 20). The malaria incidence in India currently equals approximately two million new cases per year.
In 1954, the WHO declared the goal of global yaws eradication and initiated efforts that successfully reduced the prevalence of yaws by over 95% (Antal et al. 2002). However, in the mid-1960s, following this success, the perception of yaws as a pressing public health problem waned, and advocates of integrating control efforts with other public health initiatives managed to shift resources away from yaws eradication. Hopkins (1985, p. S338) writes that "partly because of the great success of the mass campaigns of the 1950s and 1960s, the endemic treponematoses [including yaws] are widely thought to be under control" and that because they are "not fatal and usually restricted, to poor, remote, rural populations, they are not perceived to be high-priority problems by many decision makers." The global yaws status has not been well documented, but the experience in Ghana provides an example of the dramatic consequences (Figure 1(c)). In the 1970s, following a cholera outbreak that diverted attention away from yaws, incidence surged and triggered renewed campaigns to bring yaws back under control (Agadzi et al. 1983, 1985). Incidence of yaws continues to creep up with yaws now prevalent in parts of sub-Saharan Africa and Asia, although it remains largely unrecognized as a serious public health problem (World Health Organization 2009).
In the context of scarce resources available to manage diseases, competition for resources seems inevitable, even between eradicable diseases. This paper focuses on the dynamics of disease control or eradication in the context of multiple eradicable diseases and builds on our prior modeling on polio that explored the dynamics of making decisions about control and eradication (Thompson and Duintjer Tebbens 2007). That paper considered the case of polio in isolation, showing that deciding about vaccination based on cost-per-case perceptions might lead to a failure to eradicate (Thompson and Duintjer Tebbens 2007). Here, we aim to investigate how changes in perceptions of priorities might play out in the context of multiple eradicable diseases competing for resources. To model this we need to consider the impact of decisions on at least two diseases. To keep the modeling as simple as possible without limiting the possibility of observing multiple-disease behavior, we model a hypothetical population in which two eradicable infectious diseases circulate and behave according to a stochastic infectious disease transmission model, which we adapted and modified from an existing model (Edmunds et al. 1999). Assuming a fixed budget exists for vaccination against these diseases, we evaluate policies that focus...
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