 The vaccines in this study are called transmission blocking vaccines. These vaccines stimulate the persons immune system to produce antibodies against malaria. When a mosquito bites a vaccinated person, it ingests some of the persons blood. The antibodies in the ingested blood stop the malaria parasite from developing inside the mosquito. The mosquito would not be able to transmit malaria to other people. PpPfs25ISA51 Vaccine A stimulates production of antibodies against the malaria parasite Plasmodium falciparum, and ScPvs25ISA51 Vaccine B stimulates antibodies against the malaria parasite Plasmodium vivax. The vaccines also contain a substance called Montanide ISA51, which boosts the immune response to the vaccine. 