Definitions
Yekutiel (1980, 5-8) provides an excellent treatise on the concept of eradication, which includes a summary of the multiple definitions that have been formulated (Andrews and Langmuir 1963; Cockburn 1961; Cockburn 1963; Payne 1963a, 1963b; Spinu and Biberi-Moroianu 1969). A conference devoted to eradication held in Dahlem, Germany, in 1997 (Dowdle and Hopkins 1998) set out to provide precise definitions for control, elimination, eradication, and extinction in a biological, economic, and political context (Dowdle 1998, 1999; Ottesen and others 1998); however, a number of eminent public health officials (Cochi and others 1998; de Quadros 2001; Goodman and others 1998b; Henderson 1998b; Salisbury 1998) challenged these definitions at two subsequent meetings at the CDC (Goodman and others 1998a, 1998b) and the U.S. Institute of Medicine (Knobler, Lederberg, and Pray 2001).
Unfortunately, broadly accepted, standard definitions for key concepts pertaining to disease control and eradication do not exist in the literature. Making matters more confusing, certain of the concepts have been given names that are part of our everyday language and so are easily misinterpreted by nonspecialists as meaning something different from the meanings understood by those who are preoccupied with eradication programs. Most unfortunate is the all too casual use of the words elimination and eradication to promote programs that cannot reasonably be expected to achieve the promise implicit in these words. Moreover, the two words themselves are commonly used interchangeably.
Control
Two concepts are central to this chapter: control and eradication. By control, we mean a public policy intervention that restricts the circulation of an infectious agent beyond the level that would result from spontaneous, individual behaviors to protect against infection (Barrett 2004).
Although control is a range rather then a level, a particular level of control may be an aim of policy. Because every choice entails consequences, choice of the "optimal" level of control requires economic analysis. Optimal here is defined in relation to the model that gives rise to the result. Control is local and so needs to be looked at from the local perspective. Because one country's (or region's) control may affect other countries (regions), a global perspective exists as well. The level of control that is optimal for one country (region) may not be optimal from the perspective of the world as a whole. Thus, a need exists to distinguish between, say, a locally optimal level of control and one that is globally optimal.
Finally, control requires ongoing intervention. Sustaining a given level of control requires an annual expenditure.
Eradication
Eradication differs from control in that it is global. The term denotes the certified total absence of human cases, the absence of a reservoir for the organism in nature, and absolute containment of any infectious source. Eradication permits control interventions to stop or at least to be curtailed significantly. Finally, eradication is binary. Control levels can vary, but a disease is either certified as eradicated or not.
Every disease can be controlled, even if only by using simple measures, such as quarantine. The ultimate achievement of control is eradication. But not every disease that can be controlled can be eradicated. Very few diseases, in fact, are potential candidates for eradication. The criteria for the feasibility for eradication as a preference over control are discussed in the section titled "Economic Considerations."
Elimination
Control and eradication are the essential concepts, but two other terms bear mention. The first is elimination. Some who are concerned with eradication programs have explicitly defined this term to denote the cessation of transmission of an organism throughout a country or region. In contrast, eradication is defined as a global achievement. Like control, elimination is location-specific and would require ongoing interventions to be sustained in order to prevent reemergence of the disease from microbe importations.
Two problems exist with the term elimination. First, it has been used to describe different phenomena, not just that described in the definition given above. For example, some public health officials have promoted programs aimed at "eliminating a disease as a public health threat," which is interpreted to mean reducing incidence to an "acceptable" level but not necessarily to zero. This usage is very different from the one outlined above and is almost certain to be misunderstood. Second, the definition of the word elimination in common use, as applied to disease control, is indistinguishable from eradication. The 1993 edition of the New Shorter Oxford English Dictionary, for example, defines eliminate as to "remove, get rid of, do away with, cause to exist no longer." This same dictionary defines eradicate as "pull up or out by the roots, uproot, remove or destroy completely, extirpate, get rid of." This ambiguity invites misunderstanding among those not intimately involved in an eradication effort. For purposes of clarity, we seldom use the term elimination in this chapter and then only to signify control measures sufficient to interrupt microbe transmission in a specified area.
Extinction
Finally, the literature sometimes refers to extinction as a possible policy goal. In the context of infectious disease control, the concept is problematic for two reasons. First, proving that an organism has become extinct is impossible. To do so would require demonstrating not only that the organism no longer exists in nature but also that it no longer exists in any controlled environment—a practical impossibility. Second, de novo synthesis of viral agents from published genomes (Cello, Paul, and Wimmer 2002) now put the concept in peril, although much research remains to be done in this area. Extinction, in the context of infectious diseases, may no longer be irreversible.
Clearly, policy making will be improved by stating the goal of any particular intervention in precise language.
