The vaccine reduced the occurrence of severe malaria by about 30 percent, and the occurrence of severe anemia — a complication that often kills children — by about 60 percent. It did not protect well against parasite strains that were poor genetic matches , raising a concern that, over time, parasites could evolve resistance to the vaccine as they have to drugs.

Moreover, it is unclear how long even those relatively low levels of protection last; previous trials followed vaccinated children for four years. Experts also worry that parents whose children are vaccinated will become less vigilant about using mosquito nets, and less likely to seek medical care when their children develop fevers.

In 2015, Doctors Without Borders said it would not join any pilot projects such as those announced this week, arguing that its money and time would be better spent on proven malaria-fighting measures.

Nonetheless, the W.H.O.’s vaccine advisory group recommended that testing be expanded to 360,000 children to see how well the vaccine works in real-world situations, as part of routine immunization programs run by African health ministries, rather than within the careful confines of clinical trials.

The health organization’s Strategic Advisory Group of Experts will assess the results of each high-transmission season for at least three years, watching to see in particular whether parents bring in their children for all four doses and whether any rare but threatening side effects appear.

During earlier trials, some children developed high fevers and seizures soon after being immunized, but they recovered. A few children later developed life-threatening meningitis, but it was not clear that the vaccine was to blame.

The most important outcome will be overall survival rates, Dr. O’Brien said. If those rates increase substantially among vaccinated children, the W.H.O. may recommend rolling out the vaccine in more locations.