Improving the effectiveness of water, sanitation, and handwashing interventions: a simulation approach to generalizing the outcomes of intervention trials

BackgroundWhile water, sanitation, and hygiene (WASH) interventions can reduce diarrheal disease, many large-scale trials have not found the expected health gains for young children in low-resource settings. Evidence-based guidance is needed to inform interventions and future studies. ObjectivesWe aimed to estimate how sensitive the intervention effectiveness found in the WASH Benefits Bangladesh randomized controlled trial was to underlying WASH contextual and intervention factors (e.g.., baseline disease prevalence, compliance, community coverage, efficacy) and to generalize the results of the trial other contexts or scenarios. MethodsWe developed a disease transmission model to account for transmission across multiple environmental pathways, multiple interventions (water (W), sanitation (S), hygiene (H), nutrition (N)) applied individually and in combination, adherence to interventions, and the impact of individuals not enrolled in the study. Leveraging a set of mechanistic parameter combinations fit to the WASH Benefits Bangladesh trial (n=17,187) using a Bayesian sampling approach, we simulated trial outcomes under counterfactual scenarios to estimate how changes in intervention completeness, coverage, compliance, and efficacy, as well as preexisting WASH conditions and baseline disease burden, impacted intervention effectiveness. ResultsIncreasing community coverage had the greatest impact on intervention effectiveness (e.g., median increases in effectiveness of 34.0 and 45.5% points in the WSH and WSHN intervention arms when increasing coverage to 20%). The effect of community coverage on effectiveness depended on how much transmission was along pathways not modified by the interventions. Intervention effectiveness was reduced by lower levels of preexisting WASH conditions or increased baseline disease burden. Individual interventions had complementary but not synergistic effects when combined. DiscussionTo realize the expected health gains, future WASH interventions must address community coverage and transmission along pathways not traditionally covered by WASH. The effectiveness of individual-level WASH improvements will be blunted the further the community is from the high community coverage needed to achieve herd protection.