THE QUEST FOR the world’s first malaria vaccine appears to have taken a big step: A study in Africa shows experimental shots cut the risk of disease in young children by half.

The initial results from a final stage of vaccine testing were released Tuesday, and the vaccine’s developers called it a milestone in helping to tame one of the world’s most devastating killers.

However, the vaccine won’t be available for at least three years, as crucial further testing must be completed to see how well it works in infants and how long protection lasts. Then the vaccine must be reviewed by government agencies in Europe and in individual African countries.

“We still have a way to go,” Tsiri Agbenyega, lead researcher for the African study, said in a conference call with reporters.

The early results show the vaccine is only about 50 percent effective, significantly lower than the protection seen in more common vaccines. But some experts said it’s a vast improvement over the current situation, and could still save hundreds of thousands of lives.

Globally, malaria kills nearly a million people annually. More than 90 percent of them live in Africa, and most are young children and pregnant women.

Scientists have been trying for decades to develop a malaria vaccine and the one tested – developed by GlaxoSmithKline – is furthest along. Without a vaccine, efforts have concentrated on malaria drugs and other ways to prevent infection such as mosquito bed netting and insecticides.

The new vaccine targets a malaria parasite found in sub-Saharan Africa. Malaria spreads through mosquitoes, which bite people and flush malaria parasites into the bloodstream. The parasites cause bouts of high fever and can end in fatal organ failure.

The new study – still under way – began in 2009 and involves more than 15,000 children in Burkina Faso, Gabon, Ghana, Kenya, Malawi, Mozambique and Tanzania. The results focus on about 6,000 children ages 5 to 17 months. A year after three doses, the vaccinated children had about half as many cases of malaria as a group that didn’t get the vaccine.

Although there are an array of vaccines against viruses and bacteria, there has never been an effective vaccine against a parasite, which is a more complicated organism. Adding to the complexity is there are five species of malaria parasites – the new vaccine is designed specifically to protect against the deadliest one, which is common in sub-Saharan Africa.