Comparative efficacy of pre-erythrocytic whole organism vaccine strategies against the malaria parasite.

Despite major efforts over the past 50 years to develop a malaria vaccine, no product has been licensed yet. Irradiated sporozoites are the benchmark for an experimental live-attenuated malaria vaccine that induces potent protection against re-infection in humans and animal models. Lasting protection can also be elicited by parasite attenuation via tailored genetic modification or drug cover leading to renewed interest in whole-organism vaccination strategies. In this study, we systematically compared the protective efficacy of different whole-organism vaccination approaches in the Plasmodium berghei/C57bl/6 rodent malaria model. We applied blood stage parasitemia and quantitative RT-PCR of liver parasite loads as two complementary primary endpoints of a malaria challenge infection. We were able to demonstrate similar potency of genetic attenuation, i.e., uis3(-) and p36p(-) parasites, and prophylactic drug cover, i.e., azithromycin, pyrimethamine, primaquine and chloroquine, during sporozoite exposure in comparison to irradiated sporozoites. Importantly, when animals were covered with the antibiotic azithromycin during sporozoite exposure we observed superior protection. On the other end, immunizations with heat-killed and over-irradiated sporozoites failed to confer any detectable protection. Together, we show that systematic pre-clinical evaluation and quantification of vaccine efficacy is vital for identification of the most potent whole organism anti-malaria vaccine strategy.