New Delhi, July 16 -- Rain had turned the dirt road into thick mud, but Mabel Djoumessi kept walking with her 9-month-old son, Kenfack, strapped to her back. His malaria vaccination appointment at a clinic in central Cameroon was too important to miss.

For decades, millions of children across Africa have fallen sick from malaria, one of the continent's deadliest diseases for the young. But Kenfack has never had it, a feat his mother attributes to the recent arrival of a malaria vaccine.

"My other children who have never taken the vaccine frequently fall ill," Djoumessi said later, sitting with other women cradling babies at Soa District Hospital.

More than two years after Cameroon became the first country to incorporate the RTS,S malar...