NEW YORK (Reuters Health) - American researchers have reached what they hope is a milestone in AIDS research, with the launch of the first human trial of a single AIDS vaccine designed to simultaneously prevent infection with the three most common forms of HIV.

The first trial phase of the so-called "global vaccine" was launched yesterday by the National Institutes of Health (NIH) in Bethesda, Maryland, under the supervision of the Vaccine Research Center (VRC).

The trial vaccine incorporates modified parts of four different HIV genes. The genes are drawn from HIV subtype B--the most prevalent form of HIV in North America and Western Europe--as well as from subtypes A and C, which are the most common types in Africa and Asia.

Together the three types--or clades--of HIV account for about 90% of infections worldwide.

"The idea behind having this global vaccine candidate is to broaden the coverage of the vaccine," said Dr. Gary Nabel, director of the VRC. "It's giving a broader shield. And it has special relevance in the developing world, but it's also something that doesn't hurt here in North America--where we have clade B--because we don't lose anything."

Nabel pointed out that early lab tests with animals have already shown that immune response to any single type of HIV was not diminished by combining such protection against all three major HIV types. And he noted that the history of vaccine development supports the notion that such a combo vaccine can work.

"If you look at the polio vaccine, it actually contains three different strains of the disease to cover the three different most prominent strains, so there's an important precedent for that concept," he told Reuters Health.

"When we talk about developing a vaccine for AIDS, I think the one lesson we've learned with time is that this is a virus that doesn't sit still," Nabel noted. "The virus is constantly mutating, and seems to be adapting to different populations. So the idea behind having this global vaccine is that we are trying to...have a better chance of resisting the newer viruses that develop."

The trial's first phase will involve 50 healthy, HIV-negative volunteers between the ages of 18 and 40 who will receive multiple inoculations with either the test vaccine or a saline solution placebo over the course of one year.

Nabel and his colleagues expressed confidence that whether or not the vaccine proves effective over time, the vaccine itself is completely safe for use in a public trial.

"You would not get the infection from this vaccine, that's very clear," said Nabel. "It couldn't happen. We've completely modified the genes with multiple mutations to protect from any kind of activity. So that's something I can be quite definitive about.

"But," he added, "everyone who comes into this trial gets counseling about avoiding risk, because there's no proof yet that this vaccine will protect them. So they are still susceptible to natural infection."

Even if all goes well throughout the clinical trial process, Nabel noted, the vaccine will not become available to the public until at least five years from now.

Sven Bocklandt--the first volunteer to receive an injection--emphasized his hopes for the vaccine's prospects, rather than any risks it might pose.

"AIDS is a disease which personally affects me," Bocklandt--a 28-year-old NIH research fellow unconnected to the VRC project--told Reuters Health. "It's threatening my life and all the people I care about pretty much... And the only way you can eradicate it is through a vaccine. That is the only solution to the AIDS crisis. But you cannot develop a vaccine unless you test it in people and see how they respond to it. That's the bottom-line for me. So that's why I'm doing this."