Human schistosomiasis: potential long-term consequences of vaccination programmes.

Fields trials of new schistosomiasis vaccines are anticipated within the next few years, but there remains great uncertainty regarding the optimal design of vaccination programmes. Mathematical models are used here to explore the potential long-term consequences of vaccination, assuming that the vaccines provide partial protection for a limited period. The analysis suggests that vaccines acting to reduce infection rates or egg output will have a similar impact on levels of infection, that this impact may be highly sensitive to the duration as well as the degree of protection, that it may take several decades for the full impact to become apparent, and that one consequence will be peak levels of infection occurring in older age classes. In terms of lowering levels 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 be greatly increased by combining the introduction of a vaccination programme with initial mass chemotherapy. Continuous combined vaccination and chemotherapy programmes may also be more effective than either intervention alone. More research is needed on the consequences of vaccinating previously vaccinated, infected, and infected and treated individuals and the importance of natural boosting of vaccine-induced immunity.