Federal health officials are expected to announce today their approval of the first human testing of an AIDS vaccine in the United States.

The Food and Drug Administration has given approval for the testing to MicroGeneSys, a small company in West Haven, Conn., which has collaborated with researchers at the National Institutes of Health.

The vaccine will initially not be given to AIDS patients, but to uninfected volunteers to check for side effects and to determine whether the vaccine can raise an immune response in humans. Testing the vaccine for protection against AIDS infection will come later.

"It is an important moment because the sooner we get going on human trials the sooner we can get somewhere with this or other vaccines," said Dr. Allan Goldstein, leader of another vaccine research group at the George Washington University Medical School, which is also seeking approval to test a candidate vaccine.

The FDA's approval of the MicroGeneSys vaccine was first reported by Washington Drug Letter, a weekly newsletter that monitors FDA action on drugs. The Drug Letter reported in its edition yesterday that MicroGeneSys is a company with about 30 employes that was established to use genetic engineering techniques to make vaccines.

Researchers have succeeded in developing drugs that apparently help stall acquired immune deficiency syndrome in those already infected, but will not stem the spread of the virus. They have been working on developing a vaccine to prevent spread of the disease for several years, but with only moderate success.

Experiments in chimpanzees have so far been disappointing -- animals given vaccines and then injected with the AIDS virus have virtually all become infected. But because chimpanzees are not made sick by the AIDS virus, it is uncertain whether those test results will bear any similarity to what happens when humans are given the vaccine.

The first round of tests to determine side effects is expected to start by October on small groups of volunteers, most likely in six university medical centers around the country, including the University of Maryland and Johns Hopkins.

Only later will a vaccine be tested for the ability to prevent AIDS. Researchers said it is likely that in that testing phase, half of the test group will be given the candidate vaccine and half will get a placebo. The groups will be monitored to determine whether members of the vaccinated group are protected against infection.

Safety testing is expected to take six months or more, and later stages of testing could take an additional two years or more even if all goes well, researchers said. It is estimated that it could be at least five years before a vaccine is approved for general use.

Officials of MicroGeneSys were not available for comment yesterday, but in reports of the firm's work in the past year, the vaccine has been described as using proteins from the AIDS virus' outer coat.

The proteins are injected in hopes that the body will react as if they were the whole AIDS virus and thus raise a strong antibody defense against them. If the vaccine works, the antibodies could defeat the infecting virus before it could take hold.

In the MicroGeneSys work, researchers clip out the genes that make the AIDS virus' protein coat. They insert them into a virus harmless to humans, called baculovirus, which is capable of making copies of the AIDS virus coat quickly and cheaply. The proteins are then harvested and used as the vaccine.

Many laboratories have been competing to make candidate vaccines and obtain approval for human tests. All the candidate vaccines vary somewhat. Some use outer-coat proteins of the AIDS virus alone, some use proteins inserted into another virus, and some use an inner coat from the AIDS virus.

One of the chief obstacles foreseen for the MicroGeneSys vaccine is that the AIDS virus coat changes its chemical configuration continuously. So the virus can evade a vaccine against one configuration of its coat by altering chemicals.

The test of the vaccine will be the first on humans in this country. Last year Dr. Daniel Zagury, of the Pierre and Marie Curie University in Paris, immunized himself and a dozen others in France and Zaire. The test was successful in that no toxic reactions were seen, and antibodies against the AIDS virus were raised. But the strength of the immune reaction was not yet great enough to go to larger scale tests, Zagury said.