Preliminary results from a large-scale Phase III trial of RTS,S show that the malaria vaccine candidate can provide young African children with significant protection against clinical and severe malaria.

The results were announced Wednesday at a malaria forum hosted by the Bill & Melinda Gates Foundation in Seattle.

The trial, conducted at 11 trial sites in seven countries across sub-Saharan Africa, showed that three doses of RTS,S reduced the risk of children experiencing clinical malaria and severe malaria by 56 percent and 47 percent, respectively. This analysis was performed on data from the first 6,000 children aged 5 to 17 months, over a 12-month period following vaccination.

Clinical malaria results in high fevers and chills. It can rapidly develop into severe malaria, typified by serious effects on the blood, brain or kidneys that can prove fatal.

The results were also published online Tuesday in The New England Journal of Medicine. The clinical trial is scheduled to continue through 2014 and will include tests on more than 15,000 children, from infancy on up.

"This potentially translates into tens of millions of malaria cases in children being averted," Tsiri Agbenyega, who heads malaria research at a Ghanaian hospital that was one of the 11 research sites, said during a conference call from Seattle. "This is remarkable when you consider there has never been a successful vaccine against" a parasite.

"The results are encouraging, but we still have a way to go," said Agbenyega.

Half the world's population is at risk of malaria. The disease is responsible for close to 800,000 deaths each year, most of whom are children under five in sub-Saharan Africa.

It is far harder to make a vaccine against parasites like malaria than to make one against a virus. The malaria parasite changes shape as it moves from blood to liver and back to the blood, and each form has different surface proteins.