Remarks The conditionality of this recommendation is largely driven by the current higher unit cost of pyrethroid-PBO nets compared to pyrethroid-only LLINs and therefore the uncertainty of their cost-effectiveness. Furthermore, as PBO is less wash-resistant than pyrethroids, its bioavailability declines faster over the three-year estimated life of an ITN; therefore, the added impact of pyrethroid-PBO nets over that of pyrethroid-only LLINs may decline over time. The evidence comes from two sites in eastern Africa with pyrethroid resistance and not from other geographies where transmission levels and vector characteristics may vary. PBO acts by inhibiting certain metabolic enzymes, primarily oxidases, and so are likely to provide greater protection than pyrethroid-only LLINs where mosquitoes display mono-oxygenase-based insecticide resistance mechanisms. In deciding whether pyrethroid-PBO nets may be appropriate in their context, malaria programmes should:

• consider the deployment of pyrethroid-PBO nets in areas where resistance to pyrethroids in local vectors has been detected;
• determine whether resources are adequate to cover the extra cost of pyrethroid-PBO nets, while ensuring that coverage of populations at risk of malaria is not affected;
• note that WHO recommends that ITNs prequalified by WHO be selected for deployment.

Practical Info Given that the evidence indicates that unwashed pyrethroid-PBO nets are more effective than pyrethroid-only LLINs in areas with pyrethroid resistance up to 25 months post-deployment, the decision on whether to switch from pyrethroid-only LLINs to pyrethroid-PBO nets should be guided by resource availability. WHO recommends that pyrethroid-PBO nets be used where pyrethroid resistance is confirmed using standard procedures [22]. Given that pyrethroid-PBO nets are designed to provide improved impact against resistant mosquitoes in which pyrethroid resistance is, at least in part, conferred by a monooxygenasebased resistance mechanism, determining the presence of such resistance mechanisms in local vector populations will provide additional information to help target deployment of pyrethroid-PBO nets. In deciding whether to use potentially more expensive pyrethroid-PBO nets, malaria programmes should consider the impact this switch may have on vector control coverage. Deployment of pyrethroid-PBO nets must only be considered in situations where coverage with effective vector control (primarily ITNs or IRS) will not be reduced. The primary goal must be to ensure continued access and use of ITNs or IRS at levels that ensure optimal coverage for all people at risk of malaria as part of an intervention package. Post-distribution monitoring of nets to estimate coverage in terms of access to and use of nets and other malaria interventions is recommended. Pyrethroid-PBO nets should not be considered a tool that can alone effectively manage insecticide resistance in malaria vectors. It is an urgent task to develop and evaluate ITNs treated with non-pyrethroid insecticides and other innovative vector control interventions for deployment across all settings in order to provide alternatives for use in a comprehensive IRM strategy. The systematic review reported that the washing of pyrethroid-PBO nets may result in lower mosquito mortality and higher blood-feeding success than the washing of pyrethroid-only LLINs. The durability of pyrethroid-PBO nets compared to pyrethroid-only LLINs has been questioned previously based on wash-resistance data. The added epidemiological and entomological impact of pyrethroid-PBO nets depends on the bioavailability and retention of PBO on/ in the net. If this is reduced significantly over time and/or declines with washing, the greater impact of pyrethroid-PBO nets over pyrethroid-only LLINs in terms of protection against malaria may be limited to less than three years. In addition, at present, it is unknown how differences in the design/composition of pyrethroid-PBO nets affect their relative efficacy. A series of experimental hut trials with entomological end-points using non-inferiority designs have recently been completed with as a means to provide clarity in this respect [57]. As part of M&E activities, data collected by programmes on net durability would provide information on the life span of pyrethroid-PBO nets under field conditions and hence on the period over which the additional impact is maintained. Programmes that decide to switch from pyrethroid-only LLINs to pyrethroid-PBO based on concerns regarding continued effectiveness and/or insecticide resistance status of local vectors, should not revert back to the use of pyrethroid-only LLINs thereafter. Instead, programmes should plan for continued deployment of pyrethroid-PBO nets in that geographic area or develop plans for deployment of other equally or more effective new interventions once these are covered by a WHO recommendation.