An experimental vaccine can slash the risk that children will get malaria, apparently offering the first effective way to inoculate youngsters against one of the world's biggest, most intractable killers, researchers reported yesterday.

An eagerly awaited study involving 2,022 children in Mozambique, in east Africa, found the vaccine cut by one-third the likelihood of getting malaria and reduced by more than half the risk of developing serious, life-threatening cases of the disease.

"We're very excited," said Pedro Alonso of the University of Barcelona, who led the study. "This is the first conclusive evidence that a vaccine that can protect African children against malaria is possible."

While additional hurdles remain, if follow-up studies confirm the findings the vaccine could be available for widespread use within five years, marking the achievement of one of the most elusive goals in modern medicine. The effort to create a vaccine against the mosquito-borne parasitic killer has been marked by repeated failure.

"While much more work on this vaccine is still required, this constitutes a breakthrough in malaria vaccine research," said Marie-Paule Kieny of the World Health Organization.

The malaria parasite infects about 300 million people each year and kills between 1 million and 3 million, mostly children -- making it the most common infectious disease and among the top three killers. Although malaria has been largely eliminated from the United States and Europe, it remains a major public health scourge in the developing world. In Africa, malaria is the No. 1 killer of children younger than 5, claiming the life of one child every 30 seconds by some estimates.

Scientists have spent decades trying to develop an effective vaccine but have been repeatedly stymied, in part because of the complexity of the parasitic protozoan that causes malaria and its multistage life cycle. Experts had begun to wonder whether it would ever be possible to create an effective vaccine.

"Malaria has had a sense of hopelessness and intractability about it," said Melinda Moree, director of the Malaria Vaccine Initiative, which is promoting development of malaria vaccines with funding from the Bill & Melinda Gates Foundation. "These results bring hope to us all that a malaria vaccine might at last be within our grasp."

Although the vaccine so far seems far less powerful than most childhood shots, researchers said it could prove more effective when tested in younger children, who need it the most. Even if it proves no more effective, it still would provide a powerful weapon.

"It's always been thought that a malaria vaccine would not be one of these 95 percent efficacious vaccines. We think that's shooting too high and may not be necessary to really have substantial impact," Moree said. "A vaccine that could prevent half, or more than half of the severe cases of malaria, is very significant. This truly is a scientific breakthrough."

Some tentative evidence suggests the vaccine may be more effective in infants and in toddlers younger than 24 months, and researchers are planning a study to test it in that group.

"Our hope is also as we go into younger kids with the same vaccine we may also see a better effect," Moree said. "This is a vaccine that could have a significant impact on public health."

Although other experimental vaccines are being developed, the one tested in Mozambique is by far the furthest along and the first to undergo a large-scale test.

The vaccine, originally developed by the U.S. military, consists of a genetically engineered molecule that combines a key protein at one stage of the parasite's development with a protein that coats the hepatitis B virus. The combination stimulates a counterattack by the immune system.

Researchers gave 2,022 children in a rural part of southern Mozambique between the ages of 1 and 4 either three injections of the vaccine one month apart or a different vaccine, and followed the children for about six months.

The vaccine produced some pain and swelling where it was injected, but the relatively few serious side effects were far outweighed by the benefits of the vaccine, the researchers said.

Those who received the vaccine were 29.9 percent less likely to develop malaria. Children who were already infected with the parasite were 45 percent less likely to develop a second infection. And perhaps most important, the vaccine reduced by 58 percent the chance that children would develop serious, life-threatening illness, the researchers reported in a paper being published in the Saturday issue of the Lancet.

"We have very robust, statistically significant findings for both protection against clinical malaria but as well protection against infection, and even more importantly some very impressive protection against severe malaria," said Ripley Ballou of GlaxoSmithKline Biologicals, the drug company developing the vaccine. "No previously studied malaria vaccines have ever demonstrated those outcomes. I think we're talking an order of magnitude better vaccine than has ever been tested."

But even if the vaccine continues to prove effective, many obstacles remain, not the least of which would be paying for the vaccine for the millions who need it.

"It will take money. It will take resources," said Regina Rabinovich of the Bill & Melinda Gates Foundation.

GlaxoSmithKline's president and general manager, Jean Stephenne, said the vaccine could cost as much as $10 to $20 per dose, but the company left open the possibility that it could be made available at a more affordable price in the developing world.

"We expect that the price of the vaccine . . . will be reasonable and accessible to those who need it most. We intend to work actively with our partners to ensure this," the company said in a statement.