Testing vaccines in human experimental malaria: Statistical analysis of parasitemia measured by a quantitative real-time polymerase chain reaction

Clinical trials are an essential step in evaluation of safety and efficacy of malaria vaccines, and human experimental malaria infections have been used for evaluation of protective immunity of Plasmodium falciparum malaria. In this study, a quantitative real-time polymerase chain reaction was used to measure P. falciparum malaria parasitemia in non-immune volunteers who had been experimentally infected by mosquito bites. Based on a remarkably small variation in the kinetics of parasitemia, a statistical model was developed that provides detailed estimates of pre-patent periods and parasite multiplication of blood stages. Using this model, we could predict results on vaccine efficacy for 1) pre-erythrocytic vaccines in the asymptomatic incubation period and 2) asexual stage vaccines after a limited number of multiplication cycles. The model shows that stage-specific vaccines even with limited efficacy can be highly efficacious when used in combination. This P. falciparum challenge method significantly adds to the potential to evaluate efficacy of candidate malaria vaccines before going into field trials.