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,S/AS01E 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.

The RTS,S/AS01E candidate malaria vaccine is being developed for the routine immunization of infants and children living in malaria-endemic areas as part of the Expanded Program of Immunization (EPI). 30% efficacy against clinical malaria and 58% efficacy against severe malaria disease was seen with a related vaccine, RTS,S/AS02A, in children aged 1 to 4 years (Malaria-026) in Mozambique. The efficacy against clinical malaria and infection was sustained beyond 18 months. The RTS,S/AS01 vaccines have been developed in parallel with the RTS,S/AS02 vaccines, and differ in the adjuvant formulation which has been shown to be more immunogenic.The utility of a partially effective vaccination depends heavily on the overall effect of malaria incidence during a child"s acquisition of natural immunity. However, other cohorts vaccinated in Phase II studies have already discontinued follow up for episodes of malaria, and the planned Phase III studies will run for at most 30 months post vaccination. The proposed extended follow up is expected to run for four years and will inform the design of Phase 4 studies and may prove critical in informing public health policy once the vaccine is licensed.

Inclusion Criteria:

• Enrollment and vaccination in the RTS,S/AS01E clinical trial (NCT00380393)
• Written or oral, signed or thumb-printed and witnessed informed consent obtained from the parent(s)/guardian(s) of the child.

Exclusion Criteria:

• Moving out of the study area, so that follow up is impractical.

The local population is predominantly from Mijikenda ethnic group. The study area is within Kilifi District at the Kenyan coast and majority are subsistence farmers.Kilifi District experiences long rains in May-July and short rains in November/December. Measured Entomological Inoculation Rates in the area vary from 10-50 per year.