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Background  Inhibition of parasite growth is a major objective of blood-stage malaria vaccines. The  in vitro  assay of parasite growth inhibitory activity (GIA) is widely used as a surrogate marker for malaria vaccine efficacy in the down-selection of candidate blood-stage vaccines. Here we report the first study to examine the relationship between  in vivo Plasmodium falciparum  growth rates and  in vitro  GIA in humans experimentally infected with blood-stage malaria.    Methods  In this phase I/IIa open-label clinical trial five healthy malaria-naive volunteers were immunised with AMA1/C1-Alhydrogel+CPG 7909, and together with three unvaccinated controls were challenged by intravenous inoculation of  P. falciparum  infected erythrocytes.    Results  A significant correlation was observed between parasite multiplication rate in 48 hours (PMR) and both vaccine-induced growth-inhibitory activity (Pearson r = −0.93 [95% CI: −1.0, −0.27]  P  = 0.02) and AMA1 antibody titres in the vaccine group (Pearson r = −0.93 [95% CI: −0.99, −0.25]  P  = 0.02). However immunisation failed to reduce overall mean PMR in the vaccine group in comparison to the controls (vaccinee 16 fold [95% CI: 12, 22], control 17 fold [CI: 0, 65]  P  = 0.70). Therefore no impact on pre-patent period was observed (vaccine group median 8.5 days [range 7.5–9], control group median 9 days [range 7–9]).    Conclusions  Despite the first observation in human experimental malaria infection of a significant association between vaccine-induced  in vitro  growth inhibitory activity and  in vivo  parasite multiplication rate, this did not translate into any observable clinically relevant vaccine effect in this small group of volunteers.    Trial Registration  ClinicalTrials.gov [ NCT00984763 ]

Impact on Malaria Parasite Multiplication Rates in Infected Volunteers of the Protein-in-Adjuvant Vaccine AMA1-C1/Alhydrogel+CPG 7909