Efficacy of preerythrocytic and blood-stage malaria vaccines can be assessed in small sporozoite challenge trials in human volunteers.

The development of a vaccine against malaria has public health priority. In a controlled setting, preliminary data on the efficacy of Plasmodium falciparum vaccine candidates can be obtained by exposing immunized human volunteers to the bites of laboratory-reared P. falciparum-infected mosquitoes. Using empirical data, we show that these trials, with small numbers of volunteers, are sufficiently powered to detect protective biological effects induced by preerythrocytic and/or blood-stage candidate vaccines if parasitemia is measured daily by quantitative polymerase chain reaction. Sporozoite challenge trials are thus a powerful tool for early selection of candidates that warrant efficacy of trials in the field.