AIDS vaccine raising hopes in early tests

SEATTLE, Washington (CNN) -- Early data from a promising AIDS vaccine trial is raising great expectations among many AIDS researchers, scientists said Tuesday.

AIDS experts have had their eye on the vaccine developed by Merck since data from tests on monkeys was first released last year. The vaccine now is in early stage human trials, and it is sparking excitement unusual among researchers looking at such a preliminary phase.

"The level of the immune response, the percent of people who made them and the duration of the response is really quite impressive," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health.

The science behind the Merck vaccine is still being polished, but the idea was to use an adenovirus, or weakened common cold virus, to deliver an HIV gene into a human cell. A problem is many people have already been exposed to the common cold, and the immunity they have built to that would kill the version used in the vaccine, making it useless.

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Merck's Dr. Emilio Emini presented early results on efforts to work around this problem Tuesday at the ninth Conference on Retroviruses and Opportunistic Infections in Seattle. He said researchers are trying a two-prong approach: First, priming the immune system with an HIV DNA vaccine; then, boosting the initial response created by the HIV DNA with a high dose of the adenovirus vaccine that carries an HIV gene.

Currently, they are only testing one HIV gene in the adenovirus vaccine. Researchers eventually plan to add two more in hopes of creating an AIDS vaccine that would work with the most common types of HIV around the world.

The excitement about this vaccine started when data from trials with monkeys revealed that although the vaccine didn't necessarily prevent infection, none of the monkeys in the trial became sick even if they were infected.

Vaccines can trigger an antibody response or a cellular response. An antibody response kills the virus before it enters human cells so it blocks infection; a cellular response kills cells that contain the virus.

So while it doesn't block infection, it can keep an HIV infection from taking over the immune system and prevent HIV from becoming AIDS.

"We're now thinking that we'll take our victories where we can," said Dr. Gary Nabel, director of the National Vaccine Research Center at the National Institutes of Health. "If we can convert this lethal infection into a benign one, a latent infection, then that would be a major step forward."

Because the Merck vaccine creates a cellular response, Emini said, it eventually may also work as a treatment for people who are HIV-positive. HIV-infected patients are being included in the studies.

After the first phase of human tests, Merck will have to move through a second phase of testing before progressing to the large-scale testing necessary before FDA approval.

Emini said that if everything stays on schedule, researchers will be ready to move to the final phase of testing by 2004.