As the researchers report in the international open-access medical journal PLoS Medicine, most of the 30 healthy volunteers who received the vaccine developed strong, specific, and in some cases long-lasting immune responses against the MSP3 protein.
However, as researchers have learned from many early HIV and malaria vaccine trials, the crucial question is whether these immune responses mean that vaccinated people are actually protected against malaria. The straightforward way to test this is to expose the vaccinated trial participants to malaria and see if they are protected. However, this risky test is not ethical during the early stages of vaccine testing.
But Druilhe and colleagues were nevertheless able to take their early trial a step further: They demonstrated that antibodies from the blood of vaccinated volunteers were able to inhibit growth of the malaria parasite in two independent laboratory assays.
"A particular strength of the MSP3 trial is that functional assays [tests to see whether the vaccine could kill parasites] were utilized to assess the quality of the antibody response produced," say Brendan Crabb and James Beeson in an accompanying Perspective. However, they also point out that important questions remain, some of which will only be answered in additional clinical trials. Based on the results described in the PLoS Medicine article, Druilhe and colleagues have started a Phase II efficacy field trial of the MSP3 vaccine in malaria-exposed individuals.