Abstract
Herd immunity arises when a communicable disease is less able to propagate because a substantial portion of the population is immune. Nonimmunizing interventions, such as insecticide-treated bednets and deworming drugs, have shown similar herd-protective effects. Less is known about the herd protection from drinking water, sanitation, and hand hygiene (WASH) interventions. We first constructed a transmission model to illustrate mechanisms through which different WASH interventions may provide herd protection. We then conducted an extensive review of the literature to assess the validity of the model results and identify current gaps in research. The model suggests that herd protection accounts for a substantial portion of the total protection provided by WASH interventions. However, both the literature and the model suggest that sanitation interventions in particular are the most likely to provide herd protection, since they reduce environmental contamination. Many studies fail to account for these indirect effects and thus underestimate the total impact an intervention may have. Although cluster-randomized trials of WASH interventions have reported the total or overall efficacy of WASH interventions, they have not quantified the role of herd protection. Just as it does in immunization policy, understanding the role of herd protection from WASH interventions can help inform coverage targets and strategies that indirectly protect those that are unable to be reached by WASH campaigns. Toward this end, studies are needed to confirm the differential role that herd protection plays across the WASH interventions suggested by our transmission model.