ATLANTA (AP) — 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, public health efforts have concentrated on malaria drugs and other ways to prevent infection such as mosquito bed netting and insecticides.

Those efforts have been successful: Some countries have been able to reduce malaria deaths in younger children by up to 50 percent, noted Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention.

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.

In the United States, malaria has been eradicated since the early 1950s. Only about 1,500 cases are diagnosed in the U.S. each year, most of them travelers or immigrants from South Asia, sub-Saharan Africa or other places where malaria commonly spreads.

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.

Early results were released Tuesday at a malaria conference in Seattle and published by the New England Journal of Medicine.

The findings focus on about 6,000 children ages 5 to 17 months. A year after getting three doses, the vaccinated children had about half as many cases of malaria as a group that didn't get the vaccine.

Meanwhile, experts are waiting for results from a younger group — infants ages 6 to 12 weeks. That's the age when children in sub-Saharan Africa are vaccinated against other diseases. Earlier vaccination also affords earlier protection.

So far, side effects and other problems occurred at about equal rates in the vaccine and comparison groups, although higher rates of seizures and meningitis were reported in the vaccine group. The researchers think it's unlikely the vaccine was the cause, but another expert said the finding deserves more study.

The expert — Nicholas White of Thailand's Mahidol University — also questioned the release of partial results.

"It is not usual practice to publish the results of trials in pieces, and there does not seem to be a clear scientific reason why this trial has been reported with less than half the efficacy results available," White wrote in an editorial published along with the findings.

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. And 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.

Other researchers are working on different malaria vaccines, and they anticipate a need for their work even if Glaxo's continues to succeed, said Dr. Alberto Moreno, a malaria vaccine researcher at Emory University. Another deadly species more common in Asia could replace the parasite in sub-Saharan Africa as a primary killer, he said.

"We don't know what will happen in the future," he said.

No price has been set for the Glaxo vaccine. Chief executive Andrew Witty pledged the company will price it as low as possible, based on the cost of production plus 5 percent, with all the extra money plowed into research on malaria and other diseases.

"We're not going to make any money on this project," Witty said.

The Glaxo vaccine was first created in 1987. The company has invested $300 million in the vaccine so far, and expects to spend another $50 to $100 million on it, he said.

The British drugmaker paid for the study along with the PATH Malaria Vaccine Initiative, a program funded by the Bill & Melinda Gates Foundation.