A vaccine conferring immunity to only half the people receiving it is not generally considered a success, but malaria is a special case. The mosquito-borne disease caused by the Plasmodium parasite kills more than 700,000 people every year, the vast majority of them African children, and no vaccine has ever shown efficacy in trials. So in October, when drug maker GlaxoSmithKline announced that its new malaria vaccine cut African youngsters’ infection rates by about half in a large trial, global health officials rejoiced.
For years, scientists failed at developing malaria vaccines because Plasmodium changes its structure to trick the immune system. So the researchers behind the new vaccine—called RTS,S—did not formulate the vaccine just to fend off the parasite. Instead, the vaccine is designed to enhance natural immunity so the body flushes out the parasite before it gets entrenched. The vaccine contains hepatitis B antigens and protein fragments of Plasmodium falciparum, the species that causes most malarial infections. “We know the body develops a very strong immune response against hepatitis B,” says Mary Hamel, a medical epidemiologist at the Centers for Disease Control and Prevention and a principal investigator for the African trial [PDF]. “The vaccine fools the immune system into generalizing the response against malaria.”
This strategy does not always prevent infection, which explains the 50 percent failure rate. (The polio vaccine, by contrast, is more than 80 percent effective.) Even so, Hamel says RTS,S could save hundreds of thousands of lives. The trial runs until 2014, so continued success could enable children to receive immunizations by early 2015.