Fig. 3: Opsonic phagocytosis of ring-infected and uninfected erythrocytes predicts protection.

The relative phagocytosis index (RPI) for ring-infected erythrocytes (rIEs) A or uninfected erythrocytes (uEs) D was compared between plasma samples from volunteers that did (n = 56, closed circles) or did not (n = 86, open circles) require treatment after a malaria challenge. Data points represent a total of N = 142 biologically independent samples tested in a single experiment with two technical replicates. The red lines indicate the median and 95% confidence intervals. Dotted horizontal lines indicate the seropositivity level (mean plus three standard deviations of malaria-naive plasma) for phagocytosis. P value determined by two-tailed Mann–Whitney U test with no adjustment for multiple comparisons. Sub-group analysis for rIEs (B) and uEs (E) respectively for treated volunteers who either developed fever (febrile, n = 26, red circles) or did not (non-febrile, n = 30, blue triangles), and for untreated volunteers in whom parasites were either detected by PCR (PCR + ve, n = 53, purple triangles) or remained negative (PCR-ve, n = 33, orange diamonds). Data points represent a total of N = 142 biologically independent samples tested in a single experiment with two technical replicates. The black lines indicate the median and 95% confidence intervals. Dotted horizontal lines indicate the seropositivity level for phagocytosis. P values were determined by Kruskal–Wallis two-sided test followed by Dunn’s multiple comparison test. C & F Kaplan–Meier curves for the time to treatment for volunteers with a high (n = 125, black line) versus low (n = 17, red line) RPI against rIEs, and a high (n = 104, black line) versus low (n = 38, red line) RPI against uEs, respectively. A total of N = 142 biologically independent samples were analysed in n = 2 independent experiments with n = 2 technical replicates per experiment. P value determined by log-rank test in a Kaplan–Meier survival analysis and not adjusted for confounding variables. Source data are provided as a Source Data file.