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

Abstract

AbstractBackgroundRecent large-scale trials have revealed that interventions improving water, sanitation, and hygiene (WASH) in low-income settings may not confer the expected health gains for young children; evidence-based guidance is needed to inform programs and future studies.ObjectivesWe aimed to generalize the results of the WASH Benefits Bangladesh randomized controlled trial to other contexts or scenarios using a mechanistic disease transmission model and counterfactual simulations.MethodsA disease transmission model was developed 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, fidelity and adherence, and efficacy, as well as baseline WASH conditions and disease burden, impacted intervention effectiveness.ResultsIncreasing community coverage was associated with the greatest impact on intervention effectiveness (e.g., 26·9% and 52·7% median increases in effectiveness in the WSH and WSHN intervention arms when increasing coverage to 20%). The effect of community coverage on effectiveness depended on intervention completeness, i.e., the fraction of transmission that was along pathways modified by the interventions. Intervention effectiveness was reduced in counterfactual simulations with lower levels of preexisting WASH conditions or increased baseline disease burden. Individual interventions had complementary but not synergistic effects when combined.DiscussionNext-generation WASH programs must address coverage and completeness and account for the fact that effect of individual-level WASH improvements will be blunted the further the community is from achieving herd protection.