Children attending a single health dispensary (Hale dispensary, Korogwe District,Tanga Region, Tanzania) aged between 6-59 months with a fever or history of fever within the preceding 48 hours will be asked to take part. If they fit the entry criteria (see attached protocol) and consent, they will be enrolled in the standard WHO in vivo efficacy study. Asymptomatic children aged between 2 and 10 months attending the MCH clinic for immunisations and weighing will be invited to take part in the 2-10 month efficacy study and those eligible for the study and consenting will be enrolled into the study.