An experimental malaria vaccine proved highly effective in a small, early-stage clinical trial in people, raising hope in the global effort to combat the deadly disease, U.S. researchers reported on Thursday in the journal Science.

"This was something that everybody said was not possible. And here it is," Navy Captain Judith Epstein, one of the researchers, said in a telephone interview.

"We`re in the first stages now of really being able to have a completely effective vaccine," said Epstein, who said hopes to see licensing of the vaccine within three to five years.

Malaria, commonly spread by mosquitoes, infected 219 million people in 2010 and killed an estimated 660,000, according to estimates by the World Health Organization. That translates into one child in Africa dying every minute.

"It`s an important proof of concept," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said of the clinical trial held from October 2011 to October 2012.

Fauci said the test results were the most promising yet of any experimental vaccine. The vaccine was produced by privately held Sanaria Inc of Maryland.

Fauci resisted calling the trial a breakthrough. He said the test only involved a small number of people and that it was not yet clear how long the vaccine`s protection against malaria would last.

The study involved 57 healthy participants aged 18 to 45 who had never had malaria. Of these, 40 got the vaccine and 17 did not.

"There are several more steps before you can feel comfortable that you have something that might be ready for prime time," he told Reuters. "So we`re really not there yet, but it`s encouraging to see these very favorable results."

The vaccine, known as PfSPZ, is made from live but weakened parasites of the species Plasmodium falciparum, the most deadly of the malaria-causing parasites.

MANUFACTURING FEAT

To test for safety, those in the vaccine group were split into two groups who received two to six doses of the intravenous vaccine at increasing dose levels. They were followed closely for a week, and the team saw no severe side effects.

To test for effectiveness, the team exposed each study participant - those who got the vaccine and those who didn`t - to bites from five malaria infected mosquitoes.

After a week, volunteers were checked for infection, and those who were infected were treated for malaria. The team found that those who got the higher doses of the vaccine were far less likely to develop malaria that those who got lower doses or were not vaccinated.

In the study, only three of 15 participants who received higher dosages of the vaccine became infected, compared to 16 of 17 participants in the lower dosage group who became infected. Among the 12 participants who were not vaccinated, 11 became infected after exposure to infected mosquitoes.

Manufacturing the vaccine was itself an achievement. The company that produced it, Sanaria of Rockville, Maryland, was able to expose sterile mosquitoes to the malaria-infected blood, irradiate them to weaken the parasites that cause the disease and then - and this is the hard part - dissect the tiny insects to extract those parasites. Only then could they make the vaccine.

"The speed with which they were able to get enough material surprised me," Fauci said. "I thought they would never be able to do it. And they did it - and they did it very quickly and in a large amount."

The current vaccine is delivered intravenously and not through injections, which could be impractical for use in a widespread vaccination program.

"Now we`ve got to figure out a way to deliver it in a way that`s practical for mass vaccination programs," Fauci said.

Scientists from the Walter Reed Army Institute of Research and the Naval Medical Research Center participated in the study, which also holds out hope for finally shielding U.S. military personnel stationed in areas with the disease.

A statement from the Navy said malaria was responsible for "a greater loss of manpower than enemy fire in all conflicts occurring in tropical regions during the 20th century."