London, Aug 9 (PTI): A new malaria vaccine, which is being developed in the US, has shown promising results in early stage clinical trials, scientists say.

Researchers found that high doses of the vaccine protected 12 out of 15 patients from the disease. The vaccine involves injecting live but weakened malaria-causing parasites directly into patients to trigger immunity.

"We were excited and thrilled by the result, but it is important that we repeat it, extend it and do it in larger numbers," said lead author Dr Robert Seder, from the Vaccine Research Center at the National Institutes of Health, in Maryland.

Previous research has found that exposure to mosquitoes treated with radiation can protect against malaria. But studies have shown that it takes more than 1,000 bites from the insects over time to build up a high level of immunity.

A US biotech company called Sanaria took lab-grown mosquitoes, irradiated them and then extracted the malaria-causing parasite (Plasmodium falciparum), all under the sterile conditions.

These living but weakened parasites are then counted and placed in vials, where they can then be injected directly into a patient's bloodstream. This vaccine candidate is called PfSPZ.

To carry out the Phase-1 clinical trial, the researchers looked at a group of 57 volunteers, none of whom had had malaria before.

Of these, 40 received different doses of the vaccine, while 17 did not. They were then all exposed to the malaria-carrying mosquitoes, 'BBC News' reported. The researchers found that for the participants not given any vaccine, and those given low doses, almost all became infected with malaria.

However, for the small group given the highest dosage, only three of the 15 patients became infected after exposure to malaria. "Based on the history, we knew dose was important because you needed 1,000 mosquito bites to get protection - this validates that," Seder said.

"It allows us in future studies to increase the dose and alter the schedule of the vaccine to further optimise it. The next critical questions will be whether the vaccine is durable over a long period of time and can the vaccine protect against other strains of malaria," he said.

The results were published in the journal Science.