Human schistosomiasis: potential consequences of vaccination.

The potential outcomes of a Phase II schistosomiasis vaccine trial are explored using mathematical models which assume some natural immunity and a vaccine providing partial protection for a limited time. Analyses suggest that vaccination may have only a limited impact on life-long cumulative worm burdens and may lead to increased susceptibility and higher worm burdens at some ages, a "rebound" effect which results reduced opportunity to acquire natural immunity. There is a trade-off between the degree and duration of vaccine-induced protection; long-lived vaccines may be preferable to vaccines giving greater protection but for a shorter time.