Remarks DDT, which has not been prequalified, may be used for IRS if no equally effective and efficient alternative is available, and if it is used in line with the Stockholm Convention on Persistent Organic Pollutants. IRS is considered an appropriate intervention where:

• the majority of the vector population feeds and rests indoors;
• the vectors are susceptible to the insecticide that is being deployed;
• people mainly sleep indoors at night;
• the malaria transmission pattern is such that the population can be protected by one or two rounds of IRS per year;
• the majority of structures are suitable for spraying; and
• structures are not scattered over a wide area, resulting in high transportation and other logistical costs.

Practical Info Insecticide formulations currently used for IRS (24) fall into five major insecticide classes with three modes of action, based on their primary target site in the vector. These are listed below, where applicable with examples of the active ingredients contained in IRS products that have been prequalified by WHO: Sodium channel modulators

• Pyrethroids: alphacypermethrin, deltamethrin, lambdacyhalothrin, etofenprox, bifenthrin
• Organochlorines: No prequalified product available Acetylcholinesterase inhibitors
• Organophosphates: malathion, fenitrothion, pirimiphosmethyl
• Carbamates: bendiocarb, propoxur Nicotinic acetylcholine receptor competitive modulators
• Neonicotinoids: clothianidin IRS products using four of these insecticide classes have been prequalified by WHO; as of August 2020, there were no organochlorine IRS formulations prequalified (24), but DDT continues to be used in a few countries. The prequalified products have been assessed for their safety, quality and entomological efficacy, which includes evaluation of their mortality effect on mosquitoes when applied to a range of interior surfaces of dwellings found in malariaendemic areas. Residual efficacy needs to continue for at least three months after the application of the insecticide to the substrate, usually cement, mud or wood (51). Insecticides are available in various formulations to increase their longevity on different surfaces. IRS is considered an appropriate intervention where:
• the majority of the vector population feeds and rests indoors;
• the vectors are susceptible to the insecticide that is being deployed;
• people mainly sleep indoors at night;
• the malaria transmission pattern is such that the population can be protected by one or two rounds of IRS per year;
• the majority of structures are suitable for spraying; and
• structures are not scattered over a wide area, resulting in high transportation and other logistical costs. Indoor residual spraying: an operational manual for IRS for malaria transmission, control and elimination IRS is a vector control intervention that can rapidly reduce malaria transmission. It involves the application of a residual insecticide to internal walls and ceilings of housing structures where malaria vectors may come into contact with the insecticide. This operational manual (52) aims to assist malaria programme managers, entomologists and public health officers in designing, implementing and sustaining high-quality IRS programmes.