Antischistosomal vaccines: beyond the laboratory

In the effort to develop vaccines against schistosomiasis, insufficient attention has been given to the field evaluation of their safety and efficacy, and to conditions of routine use. Evaluation demands a clear case definition and precise diagnostic tools to determine the number, condition, and pathological effects of the schistosomes present in each subject. These are difficult to achieve for schistosomiasis. The trial should also assess the reduction in community transmission attributable to immunization. Ethical problems in efficacy trials include the need for subjects to be exposed to infection when means are available to minimize it, and the need to define the point at which chemotherapy must be given to subjects found to be infected. The epidemiological and economic consequences of vaccine use must be compared with alternative methods of control and with natural low-level transmission in the community. The characteristics of a desirable vaccine should be defined in advance by modelling and/or a consensus development process to provide guidance to bench researchers and to identify those candidate vaccines that should advance to field trials.