MEASUREMENT OF PLASMODIUM-FALCIPARUM GROWTH-RATES IN-VIVO - A TEST OFMALARIA VACCINES

Several prototype vaccines against the asexual blood sta of malaria ar e undergoing preclinical and phase I testing. Although these vaccines have been chosen for their ability to elicit an anti-parasite response , no practical and sensitive clinical trial procedure has been availab le for measuring their impact on parasite growth, We describe a system that allows parasite growth rates to be measured in volunteers throug h the incubation period. Two necessary elements of this system are dev eloped: suitable blood-stage Plasmodium falciparum inocula, and a high ly sensitive and quantitative assay to measure parasite growth during the incubation period. We infected five nonimmune volunteers with an i noculum as small as 300 parasites and demonstrated that the resultant in vivo asexual parasite growth rates were reproducible at 12-15-fold per cycle. The system allowed the infection to be followed for eight d ays before treatment without symptoms developing. These findings sugge st that it is feasible to directly measure the anti-parasite efficacy of a prototype malaria vaccine in human volunteers without subjecting them to the risk of disease.