Evidence for a mass community effect of insecticide-treated bednets on theincidence of malaria on the Kenyan coast

The use of insecticide-treated bednets (ITBNs) has been shown to be effecti ve in reducing mortality and morbidity from malaria. However, there is mixe d evidence as to whether or not community-wide use of ITBNs engenders a 'ma ss effect', such that those not sleeping under bednets are offered protecti on from widespread ITBN use in the area in which they live. We have analyse d data collected in Kilifi, Kenya, from a cohort of children followed from birth to investigate how the degree of net usage in the locality of a child affects the risk of developing malaria. This effect was explored using a C ox proportional hazards model. For those not using ITBNs, we found that an increasing level of ITBN usage within the area surrounding each child was a ssociated with a decreasing risk of developing malaria, thus providing evid ence in support of a mass community effect. The size and significance of th is effect were found to decrease as non-overlapping areas of increasing dis tance away from a child's home were considered. The effect was significant for areas at distances of up to 1.5 km away from each child.