The first large-scale trial of an AIDS vaccine is getting under way in South Africa, the home of two-thirds of the world's population of HIV-infected individuals. Researchers say the vaccine has shown promise in earlier studies, but in the Africa trial, they are most interested in learning whether the drug prevents spread of the disease in different parts of the world. VOA's Jessica Berman explains.

The trial is being conducted at five centers in South Africa, a country with more than five million people infected with the AIDs virus.

Three-thousand uninfected people are being recruited for participation in the four-year study. All of the volunteers will be counseled about how to prevent HIV, but investigators want to see whether those who get the vaccine, as opposed to those who get a dummy solution called a placebo, remain uninfected.

The trial is being coordinated by the Fred Hutchinson Cancer Research Center in Seattle, Washington. John Kublin is one of the study's lead investigators.

Kublin says the vaccine showed enough promise in a similar size U.S. trial for investigators to go forward with the study in Africa.

"This vaccine is resulting in very high levels of strong immune responses in those individuals who received the vaccine," he said. "And we do not know exactly if those immune responses are associated with preventing infection, or if, among those who do get infected, it helps suppress the virus."

The virus that causes AIDS is slightly different in different parts of the world. Researchers have been trying to develop a single vaccine to protect people regardless of where they live.

Kublin says investigators are extremely interested in seeing whether a promising vaccine in the United States, where the disease mostly strikes gay men, has potential in Africa, where HIV is most common among heterosexuals.

"So we are also testing the concept of whether or not one can develop a vaccine predominantly based on one strain and whether or not it will work against other strains that are not precisely matched," he said.

The vaccine uses a common cold virus to transport non-infectious viral particles into the body to stimulate the immune system against HIV.

With information gleaned from the trial in South Africa, investigators want to see how well that defense protects a diverse population against HIV, including individuals who may already be immune to the cold virus.