Efficacy-adjusted use: modelling a refined metric of insecticide treated net coverage across Africa

Insecticide-treated net (ITN) use - defined as the proportion of a population that use ITNs - is a measure of ITN uptake that is used in the estimation of malaria burden and evaluation of intervention programs. However, binary classification of individuals as users or non-users does not account for variations in ITN-related protection attributed to deleterious factors such as chemical and physical degradation, and increased insecticide resistance in vector populations. In this paper, we present a parsimonious model for malaria dynamics in mosquito-human populations in the presence of varying ITN use conditions. Using this model, we propose a new standardised measure of ITN coverage termed the "efficacy-adjusted use" defined as the equivalent level of use, assuming fully efficacious nets, that would be required to achieve the same level of theoretical EIR reduction. This more nuanced measure is used as a proxy for studying ITN-attributed protection across 44 African countries. We find that estimated protection levels in current ITN paradigms is significantly lower than indicated by crude ITN use metrics, with insecticide resistance having the largest deleterious effect. Furthermore, recent adoption of next-generation nets is estimated to have mitigated a 13% reduction in protection compared to a counterfactual pyrethroid only scenario.