Fig 1. Differential response to MDA in multi-year control study of S. mansoni in western Kenya (SCORE project [27, 29]).

School age children (SAC) ages 9–12 were screened annually in a group of 25 study villages given annual community-wide praziquantel treatment. SAC treatment coverage levels were >80% and were comparable for all villages, but the observed infection prevalence responses were highly uneven. In partitioning these 25 communities into 3 categories based on their baseline pre-intervention (Y1) SAC prevalence levels, (a) High (> 75%); (b) Medium (50% -74%); (c) Low (<50%), the uneven response was particularly noticeable in the high and medium prevalence categories. This phenomenon was not predicted by standard dynamic models [10, 17].