CHICAGO ? US researchers have discovered two powerful new antibodies which could hold the key to achieving a viable AIDS vaccine, according to a study published Thursday in the journal Science.

The antibodies are produced naturally by a minority of people infected with HIV and are able to neutralize a high percentage of the many types of the virus currently in circulation worldwide.

Researchers in California believe they can create an effective vaccine if they are able to stimulate the body to produce such "broadly neutralizing" antibodies before exposure to HIV.

"The findings themselves are an exciting advance toward the goal of an effective AIDS vaccine because now we've got a new, potentially better target on HIV to focus our efforts for vaccine design," said Wayne Koff, senior vice president of research and development at the International AIDS Vaccine Initiative.

"And having identified this one, we're set up to find more, which should further accelerate global efforts in AIDS vaccine development."

These are the first broadly neutralizing antibodies to have been identified in more than a decade and are the first from donors in developing countries, where 95 percent of new HIV infections occur.

Just four other broadly neutralizing antibodies have been discovered to date and they functioned by binding to places on the virus that have proven difficult to exploit.

"These new antibodies, which are more potent than other antibodies described to date... attach to a novel and potentially more accessible site on HIV to facilitate vaccine design," said Dennis Burton, scientific director of the vaccine initiative's California-based Neutralizing Antibody Center.

"So now we may have a better chance of designing a vaccine that will elicit such broadly neutralizing antibodies, which we think are key to successful vaccine development."

The antibodies target a region of the virus which is used to infect cells and has evolved to thwart attacks from the immune system by becoming highly variable. The antibodies appear to target regions of this protein that do not change, the study found.

This could explain their potency and breadth.

High potency is important because it allows for protection without requiring the body to produce large quantities of the antibodies.

The breadth of neutralization is important because HIV -- the virus which causes AIDS -- has evolved into so many different subtypes.

The antibodies were isolated using a new screening method which tested the blood of more than 1,800 HIV-infected volunteers from seven Sub-Saharan countries, Thailand, Australia, Britain and the United States.

Researchers are hopeful that the new method will help them discover additional broadly neutralizing antibodies.