An experimental malaria vaccine tested in varying doses provided 55 percent protection for one year to a few volunteers, a study released Monday said.

The vaccine, made by Sanaria, contains thousands of live malaria parasites weakened by radiation; the parasites create an immune response but do not persist in the body for long. The radiation is applied to mosquitoes carrying the parasites, which then are gathered from the insects’ salivary glands.

Several teams, each using a different principle, have tried for decades to develop a malaria vaccine, but complete, long-lasting protection has proved elusive.

In the current study, published by Nature Medicine, 55 volunteers were broken up into five groups who received different doses through different routes. Very high doses injected into muscles did not work. Neither did five small intravenous doses, nor three medium ones.