LONDON (November 9, 2012)--An experimental malaria vaccine that was once thought promising is turning out to be a disappointment, according to a new study that shows the vaccine 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.

Scientists also found the vaccine reduced severe malaria by about 26 percent, though its effectiveness seemed to fade after several months.

The results were announced Friday at a conference in South Africa and simultaneously published in the New England Journal of Medicine.

Dr. Jennifer Cohn, a medical coordinator at Doctors Without Borders, described the vaccine's protection levels as "unacceptably low."

She was not linked to the study.