Where and how can WASH work? Understanding limited impacts from a randomized control trial of water, sanitation, and...

Water, sanitation, and hygiene (WASH) interventions have had inconsistent benefits on diarrhea. The WASH Benefits (WASH-B) Kenya randomized controlled trial evaluated a set of WASH interventions (water W, sanitation S, hygiene H, and combined WSH) targeting children under 2 and found no reduction in diarrheal prevalence. We used a compartmental model fit to WASH-B Kenya data (n = 11,856) and explored whether and how changes to intervention factors (intervention efficacy, compliance, and community coverage) and contextual factors (transmission intensity, baseline WASH conditions, completeness of transmission pathways targeted) could reduce child diarrhea by 50% compared to the control arm. We estimated that this target could not be achieved by single-intervention arms (W, S, or H). However, it could be achieved in the WSH arm if there were substantial intervention improvements (i.e., 50% intervention efficacy, 100% compliance, and 60% community coverage). High baseline transmission intensity and limited preexisting WASH infrastructure in Kenya hindered impact, so we also considered changing contextual factors could influence achievable benefits. Improvements in contextual factors coupled with more modest increases in intervention factors could achieve a 50% reduction in diarrhea in the W, H, and WSH arms. Thus, in settings like Kenya with high diarrhea prevalence, WASH interventions must be used by most of the population (>60%) and block multiple transmission routes (as in the WSH arm) to appreciably reduce burden. In settings with more favorable contextual factors, diarrhea can be reduced with moderate intervention factor targets. Applications of this approach could inform WASH policies, programs, and trial design.