Figure 4.

Influence of parasite exposure during immunization on the magnitude of the antibody response and protection status. A and B, Chemoprophylaxis and sporozoite (CPS)–immunized volunteers from study B protected from mosquito challenge after exposure on 3 occasions, to bites from 15 (3 × 15; orange; n = 4), 10 (3 × 10; yellow; n = 8), or 5 (3 × 5; white; n = 5) Plasmodium falciparum–infected mosquitoes. Circumsporozoite protein (CSP)- and merozoite surface protein 1 (MSP-1)–specific antibody levels (in arbitrary units [AU]; A) and blood-stage parasitemia (expressed as the number of P. falciparum–parasitized erythrocytes per mL of blood, determined by quantitative polymerase chain reaction; B) were analyzed 28 days after the first (I), second (II), and third (III) CPS-immunization. C and D, 3 × 5 CPS-immunization did (white; n = 5) or did not (purple; n = 5) result in protection from subsequent mosquito challenge. C, CSP- and MSP-1–specific antibody (in AU) and (D) erythrocytic parasitemia (in P. falciparum/mL) 28 days after immunizations I, II, and III are displayed. E, Correlation between CSP and MSP-1 antibody responses (in AU) 28 days after immunization III and cumulative parasitemia (in P. falciparum/mL) over the course of all CPS-immunizations for all study B volunteers. All antibody responses were corrected for the volunteers’ background response before immunization and are presented as individual values (dots) and whisker box plots (box, median with 10th–90th percentile; whiskers, minimum to maximum) and individual values (dots). Differences between groups over time were analyzed by repeated-measures, mixed-model 2-way analysis of variance with the Bonferroni post hoc test. Correlation was analyzed by the Spearman coefficient (r). *P < .05, **P < .01, and ***P < .001.