Rebound mortality and the cost-effectiveness of malaria control: potential impact of increased mortality in late childhood following the introduction of insecticide treated nets.

The efficacy and relative cost-effectiveness of insecticide-treated nets (ITNs) for the control of malaria in children under 5 years of age have recently been demonstrated by several large-scale trials. However, it has been suggested that long-term use of ITNs in areas of high transmission could lead to mortality rebound in later childhood, which would reduce the cost-effectiveness of the intervention, and at the extreme could lead to negative overall effects. A model is presented in which the cost and disability adjusted life years (DALYs) per child aged 1-119 months were estimated for a sub-Saharan African population with and without an ITN intervention. The rebound rate, defined as the percentage increase in age-specific all-cause mortality and malaria specific-morbidity, was varied to estimate the threshold at which the intervention was no longer cost-effective. Rebound was considered over two possible age ranges: 5-9 years and 3-6 years. With mortality and morbidity reductions due to ITNs in children aged 1-59 months and rebound in the 5-9 years age class, one could be reasonably certain that the cost per DALY averted is below $150 up to a rebound rate of 39%. Up to an 84% rebound rate it is highly likely that the intervention will be DALY-averting, that is the DALYs averted by the intervetion outweigh DALYs incurred through rebound effects. These thresholds are sensitive to the age range over which reductions and rebound in morbidity and mortality occur. With reductions confined to children aged 1-35 months and rebound in the 3-6 years age class, the cost per DALY is highly likely to fall below $150 only up to a 2.5% rebound rate, and with a rate in excess of 11% one can no longer be reasonably certain that the intervention is DALY-averting. These rates apply to the whole population. If there is no rebound amongst children who did not comply with the intervention, the actual increases in morbidity and mortality required to reach these thresholds amongst compliers would be much higher. The age range over which rebound occurs is a critical determinant of the thresholds at which one can no longer be reasonably certain that ITNs remain cost-effective in the long term. Based on empirical estimates of age-specific malaria mortality in sub-Saharan Africa, it appears unlikely that this threshold rate would be reached if rebound occurs over the 5-9 years age range. By contrast, if rebound occurs over the ages of 3-6 years, the increase in mortality rates required to reach this threshold falls within the observed range of malaria-specific mortality rates for this age group. It is essential that long-term surveillance is included as part of ITN interventions, with particular attention to the age range over which rebound may occur.