Parasite clearance rate depends on the efficacy of antimalarial drug and host immunity. Host immunity depends on age - immunity in infants is likely to be lower than the immunity in 1-4 year old children. Parasite clearance rate depends on parasite density, multiplicity of infection, and the type of host response - these factors differ between clinical episodes of malaria and asymptomatic parasitaemia. The interaction between antimalarial drug and host response during a clinical episode of malaria is likely to be different from that during asymptomatic malaria parasitaemia. Although infants with mild illness will not be excluded from IPTi administration at routine EPI clinics, the majority of infants will be asymptomatic when they receive IPTi. Thus results of the standard WHO in vivo drug sensitivity studies conducted in 6-59 month-old symptomatic children are unlikely to be applicable to 2-10 month old mostly asymptomatic infants receiving IPTi. However, there is no empirical evidence to support or refute this assertion.