Refining the resistance-to-damage (RD) score to predict operational Insecticide-Treated Net lifespan and identify paths to innovation

BackgroundInsecticide Treated Nets (ITNs) vary greatly in their physical characteristics yet the relationship with field durability remains poorly understood. To address this, we refined the existing resistance-to-damage (RD) metric for greater predictive power in estimating operational lifespan and to identify key avenues of innovation. MethodsThe original resistance-to-damage (oRD) score was calculated by comparing four physical attributes; snag strength, bursting strength, hole enlargement resistance and abrasion resistance; against aspirational values previously determined to provide resistance to routine damage. Here, the weighting of each is improved to provide a better prediction of field life, resulting in a weighted resistance-to-damage (wRD) score. Durability data from 37 field sites (across 14 countries) involving 10,000 campaign ITNs was used to quantify the relationship between physical characteristics and field life. Bivariable linear regression models were used to assess the correlation between RD scores and ITN survival time. A Cox regression survival analysis was applied to examine the impact of the RD score on physical survival, adjusting for net care and environment variables (Risk Index), evaluating the risk of ITN failure from the time they were first hung. ResultsThe bivariate analysis, not adjusting for differences between sites, shows that the wRD was a better predictor for net lifespan than oRD (wRD r2= 0.07 oRD r2= 0.01). In the multivariate analysis, adjusting for risk index, the wRD model was able to explain more than 70% of variability in net survival (wRD r2 = 0.74). Consequently, when comparing nets with different wRD scores in the same site, a 10-point increase in wRD score was associated with a 3.9-month gain in median survival (p=0.016). Increasing wRD from 35 to 70, the range of current products, increases median field life by 13.65 months (95% CI: 3.03-24.16). An ITN product with an wRD score of 68 is associated with a median lifespan of 2.7 years. The increase in predictive power of the wRD score came primarily from increasing the weighting of the hole enlargement resistance variable, which is a strong differentiator between ITN products. In contrast, abrasion-resistance was a negligible contributor to predictive power. DiscussionThe wRD score more accurately predicts ITN survival in the field than the oRD score. These findings highlight that resistance to hole enlargement is the primary differentiating factor in predicting field life, with some products performing far better than others. Snag and bursting strength remain essential characteristics yet are similarly low compared to aspirational values across available ITN products. All ITNs WHO prequalified in 2020 perform poorly on snag strength, indicating this as a core bottleneck in achieving longer operational ITN lifespan. Consequently, resistance to snagging and hole enlargement should be keys targets for future innovation to enhance the physical durability of ITNs.