Durability of the insecticidal activity of next-generation insecticide treated nets distributed for malaria control in Mozambique: findings from the New Nets Project (2020-2022)

BackgroundAs next-generation insecticide-treated nets (ITNs) are increasingly deployed, it is essential to monitor their durability over time under operational conditions to support evidence-based procurement and replacement strategies by national malaria programs. This study evaluated the insecticidal durability of next-generation ITNs distributed in Mozambique through the New Nets Project (2020-2022), with a focus on bioefficacy and chemical content over 24 months of household use. MethodsAn ITN insecticidal durability study was conducted with nets from four districts of Mozambique to evaluate four ITN product types: pyrethroid-only (MAGNet(R), DuraNet(R), Olyset(R) Net), pyrethroid-PBO (Olyset(R) Plus), pyrethroid-pyriproxyfen (Royal Guard(R)), and pyrethroid-chlorfenapyr (Interceptor(R) G2). ITNs were withdrawn from households at 6, 12, and 24 months post-distribution. Bioefficacy was assessed via WHO cone and tunnel bioassays using insecticide-susceptible and pyrethroid-resistant laboratory strains of Anopheles gambiae s.l., and through experimental hut trials in Cove, Benin against wild, pyrethroid-resistant An. gambiae s.l. populations. Chemical analysis was conducted to quantify active ingredient content over time. ResultsThe pyrethroid components of all ITN types retained high efficacy, with 100% of nets meeting WHO bioefficacy criteria at 6, 12, and 24 months in bioassays using the susceptible An. gambiae KISUMU strain. However, the proportion of Olyset(R) Plus nets meeting WHO thresholds in bioassays assessing the bioefficacy of the PBO component against the pyrethroid-resistant An. gambiae s.l. AKRON strain, declined from 73% at 6 months to 40% at 24 months. Royal Guard(R) showed variable performance for pyriproxyfen bioefficacy, with pass rates increasing from 43% to 87% over the same period. Interceptor(R) G2 maintained high bioefficacy against the pyrethroid-resistant VKPER strain, with [≥]92% of nets meeting WHO efficacy criteria across all time points. Experimental hut trials confirmed the superior field performance of Interceptor(R) G2 compared to pyrethroid-only nets, though mortality declined from 55% to 39% and blood-feeding inhibition from 72% to 19% between 6 and 24 months. Chemical analysis showed substantial degradation of PBO and chlorfenapyr by 24 months, with residual levels at 8% and 32% of baseline, respectively. ConclusionNext-generation ITNs provide improved protection against pyrethroid-resistant malaria vectors, but the durability of their enhanced efficacy varies by active ingredient. Interceptor(R) G2 demonstrated the most consistent performance over 24 months, reinforcing its utility for control of pyrethroid resistant malaria vectors. However, declining personal protection and insecticide content underscore the need for long-term monitoring. These findings support the implementation of extended, context-specific durability studies to guide ITN selection, replacement strategies, and policy decisions for sustained malaria control.