Anti-schizont antibody ELISA absolute OD readings measured at screening. Serum diluted 1:1,000. Negative controls = OD readings from UK malaria-naïve adults (n = 30). Positive controls = OD readings from hyperimmune Kenyan adults living in malaria-endemic regions (n = 6). Minimal exposure = subjects enrolled in Groups 1, 3 and 5 (n = 14). Definite exposure = subjects enrolled in Groups 2, 4 and 6 (n = 14). Screened subjects = all volunteers that had blood drawn at screening (n = 145). Median values represented by lines through each dataset.