 Malaria is one of the leading causes of deaths in children below five years old. Despite antimalarial drugs and insecticide treated bed nets, the established means of treatment and protection, malaria still continues to affect many children. A malaria vaccine would be a very effective way of reducing malaria infection in the community. RTS,SAS01E is a leading malaria vaccine candidate which is being developed for children in Africa. The investigators have done a study to find out if this vaccine is effective in reducing infection by malaria in children aged 5-17 months living in Kenya and Tanzania. The follow up of the original study was 14-18 months. The extended follow up is proposed to continue for another four years. 