The trial is also being funded by the South African AIDS Vaccine Initiative and the HIV Vaccine Trials Network.

But "a vaccine is years and years away," he told United Press International.

The vaccine candidate, produced by Merck, consists of a cold virus with three HIV genes and will be tested on 3,000 healthy men and women ages 18 to 35 at five South African sites.

Ideally, exposure to the genes will create an immune response strong enough to ward off infection with the actual HIV virus during future exposure, said study co-director James Kublin, a faculty member at the University of Washington.

Preventing HIV transmission, however, has proven difficult, Kublin told UPI, so the trial will be testing to see if the vaccine stops the infection -- or even just slows it down.

"In the case of HIV, we've been humbled over the last 20 years," he said. "In individuals who become infected, we may be able to control the infection so that it is less severe."

The process of operating any large-scale drug trial is arduous, but when the trial involves AIDS, it poses additional challenges, experts say.

Recruiting the large numbers of volunteers required is difficult, Kublin said, but they also must individually be informed about the nature of randomized trial with a placebo. Because of the sensitive nature of the subject, community education efforts are also under way.

Once patients are part of the trial, they are counseled about safer sex practices and supplied with condoms.

"The incidence of HIV is generally lower among study participants than non-participants because they are routinely counseled and given condoms," he said.

The complicated nature of testing, not just for HIV transmission but also immune response, requires sophisticated lab equipment and experienced workers, Kublin added.

But perhaps the largest challenge of all is the nature of the disease itself.

HIV reproduces and mutates quickly, changing its outer surface to "cloak itself in the body of an infected person," he said. It also hides in the immune system -- the very system that would fight against it.

There are also different types or "clades" of HIV infection, said Fauci.

The new trial will test whether a vaccine against clade B, which is the most prevalent in the developed world, will also help prevent transmission of clade C, which is the most common type in Africa.

"This is another in a series of steps and trials evolved over the last few years asking important questions," Fauci said.

There are more than 20 vaccines in various stages of the research process, Wayne Koff, senior vice president of research and development at the International AIDS Vaccine Initiative, told UPI. Of those 20 candidates, several have advanced past early-stage safety trials and are also being tested for their protection against HIV.

All of the candidates, however, have one thing in common.

Vaccines for measles and polio both stimulate antibodies that destroy virus cells. In the case of AIDS, no researcher has successfully developed a potential vaccine to produce that response, Koff said.

Instead, all the current vaccine candidates stimulate T-cells, which kill human cells containing the virus before they can replicate and spread the disease within the body. Potential vaccines are ranked based on how much of a T-cell response they evoke.

Given the reliance on T-cell response, it could make a difference whether an individual has been exposed to the particular cold virus used for a vaccine. If that turns out to be true, that could be one of the most important pieces of information to emerge from the trial, Koff said.

"That's one of the issues the field of HIV research is looking at really critically at this time."

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