Practical Info Mosquito nets that include both a pyrethroid insecticide and the synergist PBO have become available. PBO acts by inhibiting certain metabolic enzymes (e.g., mixed-function oxidases) within the mosquito that detoxify or sequester insecticides before they can have a toxic effect on the mosquito. Therefore, compared to a pyrethroid-only net, a pyrethroid-PBO net should, in theory, have an increased killing effect on malaria vectors that express such resistance mechanisms. However, the entomological and epidemiological impact of pyrethroid-PBO nets may vary depending on the bioavailability and retention of PBO in the net, and on the design of the net (i.e. whether only some or all of the panels are treated with PBO). At present, it is unknown how these differences in the design/composition of pyrethroid-PBO nets affect their relative efficacy. A noninferiority design for experimental hut studies with entomological endpoints is being explored by WHO as a means to provide clarity in this respect.