FDA APPROVES 1ST TEST OF AIDS VACCINE.

Byline: Lawrence K. Altman The New York Times

The Food and Drug Administration has given a California company approval to conduct the world's first full-scale test of a vaccine to prevent infection with the AIDS virus, the company announced Wednesday.

The experiment is to involve 5,000 uninfected individuals in up to 40 clinics in the United States and Canada and 2,500 volunteers in 16 clinics in Thailand over the next four years, the company, VaxGen Inc. of South San Francisco, said. It said it expects to receive approval from Thai health officials to begin testing in that country later this year.

The announcement brought expressions of cautious hope among health officials and advocates for people with AIDS. HIV, the AIDS virus, has infected an estimated 30 million people in the world, and many experts say it will take a vaccine to stop the worsening epidemic.

But scientists are sharply divided over when and which experimental vaccines to approve for full-scale testing. No vaccine is 100 percent effective. Some experts favor testing any promising vaccine, even if it is likely to protect only a small proportion of recipients, arguing that something is better than nothing in a health emergency.

But other experts say that full-scale tests should be conducted only when laboratory and animal research shows a solid scientific understanding of how an experimental vaccine works.

The VaxGen vaccine was safe in earlier tests involving 1,200 volunteers beginning in March 1992 and induced antibodies in more than 99 percent of the vaccinated participants, the company said. The major question the new tests aim to answer is how effective the experimental vaccine will be among people who are exposed to HIV because of high-risk sexual practices or injecting drugs.

Advocates for people with AIDS and many scientists expressed hope that the vaccine would protect a large proportion of recipients.

``This is a watershed event in the AIDS crisis,'' Dr. Seth Berkley, president of the International AIDS Vaccine Initiative, a nonprofit group that seeks to accelerate the development of safe and accessible AIDS vaccines.

But Berkley said he expected that a number of additional trials in developed and developing countries would be needed to test the safety and effectiveness of a number of anti-HIV vaccines.

Daniel Zingale, a spokesman for the AIDS Action Council, said his Washington-based national advocacy organization was pleased at the FDA action and ``cautiously hopeful'' that it would work.

Other scientists expressed doubts about the vaccine's prospects, saying that the earlier tests showed it boosted only one part of the immune system involving antibodies, and thus is unlikely to protect substantial numbers of recipients. Antibodies are proteins are formed to destroy invading germs in the blood.

Dr. David Baltimore, a Nobel Prize winner and head of a government advisory committee on AIDS vaccines, said that it was ``unlikely that the vaccine would be totally protective.''

The main reason is that most experts believe an anti-HIV vaccine needs to boost another arm of the immune system known as killer T-cells that destroy virus-infected cells, said Baltimore, who is president of the California Institute of Technology.

Baltimore questioned whether the trial's design could determine whether illness was less severe among those vaccinated, if they became infected. He called on VaxGen to disclose the trial's scientific blueprint. VaxGen officials did not respond to a call asking for it.

In the trial, three injections of the genetically engineered vaccine, known as Aidsvax, will be given to each volunteer over several months. The vaccine is made from a portion of HIV's outer coat, known as gp120, that attaches to cells to infect them.

The first shots are expected to be given in about a month in the United States. For participants, the company is seeking gay men and the uninfected partner among discordant couples. In Thailand, the participants will be uninfected injecting drug users.