MALAVA, KenyaOn a sunny morning outside a hospital in western Kenya, 20 or so mothers sit on wooden benches with babies in their arms. Wrapped in colorful blankets and kitenge pattern cloths, one by one the babies are injected with a shot. And one by one, they cry. Pole baby—sorry, baby—says the nurse. But the prick is worth the pain: These babies are among the earliest to receive the first-ever vaccine against malaria, one of the deadliest diseases in the world.

Hundreds of thousands of children across malaria-stricken regions of Kenya, Malawi, and Ghana are receiving the RTS,S malaria vaccine, which Western health experts laud as an exciting new tool in the global fight against the disease. But after 35