Durability of the insecticidal activity of next-generation insecticide treated nets distributed for malaria control...

As next-generation insecticide-treated nets (ITNs) are increasingly deployed, monitoring their durability under operational conditions is critical to inform procurement and replacement decisions by national malaria programmes. This study evaluated the insecticidal durability of next-generation ITNs distributed in Mozambique through the New Nets Project (2020-2022), focusing on bioefficacy and chemical content over 24 months of household use. ITNs were collected from four districts in Mozambique and included pyrethroid-only nets (MAGNet, DuraNet, Olyset Net), pyrethroid-PBO nets (Olyset Plus), pyrethroid-pyriproxyfen nets (Royal Guard), and pyrethroid-chlorfenapyr nets (Interceptor G2). Nets were withdrawn at 6, 12, and 24 months post-distribution. Bioefficacy was assessed using WHO cone and tunnel bioassays with insecticide-susceptible and pyrethroid-resistant laboratory strains of Anopheles gambiae s.l., alongside experimental hut trials in Covè, Benin using wild pyrethroid-resistant mosquito populations. Chemical analysis quantified active ingredient content over time. All net types retained high efficacy against susceptible mosquitoes, with 100% meeting WHO bioefficacy thresholds at all time points. However, Olyset Plus efficacy against resistant mosquitoes declined from 73% at 6 months to 40% at 24 months. Royal Guard showed variable pyriproxyfen performance, improving from 43% to 87%. Interceptor G2 maintained consistently high efficacy against resistant mosquitoes, with ≥92% of nets meeting WHO criteria across all time points. Experimental hut trials confirmed superior field performance of Interceptor G2 compared with pyrethroid-only nets, although mortality declined from 55% to 39% by 24 months. Chemical analysis showed major losses of PBO and chlorfenapyr, with only 8% and 32% remaining at 24 months, respectively. These findings demonstrate substantial variation in the durability of next-generation ITNs and support prioritising pyrethroid-chlorfenapyr nets for malaria control programmes in areas with pyrethroid resistance.