Malaria remains a major public health threat despite regulatory approval of a partially effective pre-erythrocytic malaria vaccine. There is an urgent need to accelerate the development of a more effective multi-stage vaccine. Controlled human malaria infection (CHMI) has been shown to be an important tool for the assessment of the efficacy of novel malaria vaccines and drugs prior to field trials. CHMI also allows for the evaluation of immunity to malaria and parasite growth rates in vivo. This is particularly useful in individuals from endemic areas with a level of exposure and immunity to malaria. Thus CHMI in individuals with prior exposure to malaria could be a valuable tool to accelerate malaria vaccine development. In this study, the investigators aim to use CHMI in semi-immune adults to provide a comprehensive prioritization of antigens associated with blood-stage immunity for vaccine development. The investigators will comprehensively characterize immunity to malaria using >100 antigens in up to 2,000 semi-immune adults, from known areas of malaria endemicity in Kenya, then select 200 individuals with a range of different immunological profiles, and conduct CHMI studies with serial quantitative polymerase chain reaction (PCR) to measure the parasite growth rate in vivo and relate this to host immunity. This will also involve analysing the relationship with functional immunity assessed by laboratory assays.