HUMAN SCHISTOSOMIASIS - POTENTIAL LONG-TERM CONSEQUENCES OF VACCINATION PROGRAMS

Field trials of new schistosomiasis vaccines are anticipated within th e next few years, but there remains great uncertainty regarding the op timal design of vaccination programmes. Mathematical models are used h ere to explore the potential long-term consequences of vaccination, as suming that the vaccines provide partial protection for a limited peri od. The analysis suggests that vaccines acting to reduce infection rat es or egg output will have a similar impact on levels of infection, th at this impact may be highly sensitive to the duration as well as the degree of protection, that it may rake several decades for the full im pact to become apparent, and that one consequence will be peak levels of infection occurring in older age classes. In terms of lowering leve ls of infection there may be advantages in delaying vaccination until children reach school age, especially if the vaccine gives short-lived protection, or to repeat vaccination. The short-term advantages can b e greatly increased by combining the introduction of a vaccination pro gramme with initial mass chemotherapy. Continuous combined vaccination and chemotherapy programmes may also be more effective than either in tervention alone. More research is needed on the consequences of vacci nating previously vaccinated, infected, and infected and treated indiv iduals and the importance of natural boosting of vaccine-induced immun ity. (C) 1997 Elsevier Science Ltd.