Discussion
This paper describes the methodology for evaluating the physical and insecticidal durability of two dual AI ITNs (InterceptorDual AI ITNs cost significantly more than standard pyrethroid nets due to the additional non-pyrethroid active ingredient on these nets. To ensure value for money, it is essential to closely monitor the insecticidal activity of the non-pyrethroid component in these nets using insecticide durability bioassays. Current WHO ITN insecticide durability guidelines were developed mostly for pyrethroid-only nets hence the methods and recommended strains were suited to their rapid mode of action [For InterceptorFor Royal GuardCurrent WHO ITN evaluation guidelines define specific thresholds for determining the efficacy of ITNs in insecticidal durability bioassays. Nets that induce ≥ 95% knockdown and/or ≥ 80% mortality in cone bioassays or ≥ 80% mortality and/or ≥ 90% blood feeding inhibition in tunnel tests are considered efficacious. While proposing these same efficacy criteria for the laboratory bioassays for alpha-cypermethrin and chlorfenapyr bioefficacy in the study nets, it is recognized that these cut-offs were primarily developed for fast-acting pyrethroid insecticides on nets and may need to be modified for non-pyrethroid insecticides. Indeed, new draft WHO guidelines proposed for ITN prequalification aim more for a weight of evidence approach and do not allude to specific efficacy thresholds [Despite the methodological modifications we propose for laboratory bioassays in this study, it is unclear whether typical ITN laboratory bioassay techniques (cone bioassays and tunnel tests) would be sufficiently informative of the insecticidal durability of dual AI ITNs as these methods were developed for pyrethroid-only ITNs [In standard ITN experimental hut evaluations, the performance of nets washed 20 times is widely accepted as a measure of their efficacy after 3 years of field use and ITN products have been prequalified by WHO based on their performance after 20 washes. However, not much is known about how this washing process and number of washes relates to the natural ageing of nets in households. The hut trial comparing naturally aged nets to washed nets in our study provides an opportunity to investigate this claim. To further support this objective, the number of times householders’ wash their nets over 3 years of household use and the washing methods used will also be investigated. Another key objective of this study is to use mathematical models to determine whether the entomological performance of ITNs in the experimental hut trials performed in this study could be predictive of their performance in the cRCT. The findings from this modelling study will provide further insights into the role of experimental hut trials in the dual ITN product development process.