Relative versus absolute risk of dying reduction after using insecticide-treated nets for malaria control in Africa.

Four recently completed large-scale randomized controlled trials have assessed the impact of insecticide-treated bednets and curtains on overall child mortality in Africa. These results have sparked numerous discussions among implementing agency representatives and researchers about the public health significance of the results. For the interpretation of impact, most of the arguments have been based on the observed decrease in protective efficacy (a relative measure of impact) with increasing malaria transmission (range: 14-29%, regression for trend: F = 245 on 1,2 DF, P < 0.003). However, an analysis of the absolute measure of impact (the risk difference) showed a different pattern. The impact ranges from 3.8 to 6.9 lives saved per 1000 children protected per year, without a significant trend (F = 2.8 on 1,2 DF, P = 0.2) and with equally high values in both low and high transmission sites. When assessing the public health importance of an intervention, both relative and absolute decrease in risk should be considered.