LONDON — An experimental malaria vaccine once thought promising is turning out to be a disappointment, with a new study showing it is only about 30 percent effective at protecting infants from the killer disease.

That is a significant drop from a study last year done in slightly older children, which suggested the vaccine cut the malaria risk by about half — though that is still far below the protection provided from most vaccines. According to details released on Friday, the three-shot regimen reduced malaria cases by about 30 percent in infants aged 6 to 12 weeks, the target age for immunization.