When should cholera vaccine be used in cholera-endemic areas?

Abstract

The paper by Vu Dinh Thiem et al. in this issue of the Journal provides valuable documentation of the costs for providing killed oral cholera vaccine during a mass-vaccination programme in Vietnam (1). Vietnam is the only country where cholera vaccine is now being given to endemic populations, although many other countries in Asia and Africa have regular seasonal outbreaks of cholera. This raises the question of defining the circumstances for using cholera vaccine in endemic areas. The World Health Organization (WHO) has recommended use of killed oral cholera vaccine in refugee populations of Africa (2), but it has not yet formulated a policy for use of either the live or killed oral vaccines in areas endemic for cholera (Chaignat CL. Personal communication, 2003). There are several reasons why it has been difficult for WHO to formulate such a policy. The true burden of disease from cholera is not well-understood since many nations with cholera do not report it because of fears of restriction on trade and travel, and when they do, the numbers likely do not reflect the true rates of infection. Outbreaks or epidemics are more likely to be reported than cases occurring in endemic situations, and many endemic areas do not have the laboratory or epidemiological surveillance resources to document accurate rates. Since countries are not reporting cholera, it becomes difficult for WHO to recommend a vaccine for an infection that is not recognized to occur (3). However, many geographic areas continue to have a large burden of disease from cholera, often exceeding one hospital case per thousand annually. In these areas, cholera is well-known to the local population, they greatly fear it and would likely welcome a vaccine providing protection. Since the recommendations differ between refugee situations and endemic areas, it would be useful to review some differences in these two situations (Table). In endemic areas, people have some degree of acquired immunity, and this pre-existing immunity modulates the effectiveness of any vaccine. It is also possible to establish surveillance for cholera and, thus, to determine a true burden of disease. When patients develop symptoms of cholera, they use the routine primary healthcare facilities available for any other diarrhoeal disease, including government hospitals or traditional providers. In either case, the decision for implementing a cholera vaccine programme would logically depend on an analysis of its cost-effectiveness from a national perspective. Because the patients are using routine facilities, there is likely to be less public awareness of the problem, and it would be handled as a national problem, with little opportunity for urgent international funding. Thus, the funds to deal with the problem will need to come from a regular budgetary provision. By contrast, the situation of cholera outbreaks in refugees is quite different. Refugees often have little or no immunity since they may have migrated into a cholera area from one that was not cholera-endemic. By definition, there has been no background information on expected disease incidence since there has been no surveillance. Based on other similar situations, however, rates might be expected to be high (4). In contrast to the endemic area, medical care is often provided by special facilities for refugees, and these facilities are often provided by international agencies. These agencies are often well-connected to journalists who may publicize the cholera epidemics, and this publicity may generate substantial emergency funds from international donors. Cost-effectiveness considerations may then take a secondary role to the emergency humanitarian need from the epidemic. The experience of ICDDR,B in Bangladesh might be cited to illustrate the situation in a cholera-endemic area. In its rural Matlab field area with a population of over 200,000, the annual incidence has generally been from 1 to 5 case(s) of cholera per 1,000 as detected by hospital surveillance (5-7). …