Discouraged by initial failures to produce an effective vaccine against AIDS, American scientists have shifted their focus to finding drugs that can halt the progression of the deadly virus.

Vaccines, which produce antibodies that prevent infection from a disease, have been considered the best hope to stop the spread of AIDS, and the search for them continues.

But at a conference that began yesterday at the National Academy of Sciences' Institute of Medicine, key researchers said that developing potent new drugs to treat the 2 million Americans believed to be infected with the virus should take precedence.

"The vaccine effort is running into serious problems," said David Baltimore, director of the Whitehead Institute at MIT and a leading AIDS researcher.

"Last year we estimated a minimum of five years. Things today, if anything, seem bleaker than that," he said.

Other speakers, citing the complexity of the virus, its remarkable ability to disarm the human immune system, and its capacity to attack the central nervous system in addition to blood cells, concurred with Baltimore. Most vaccine research so far has been carried out with laboratory animals.

"Clearly the chimpanzee work has no necessary relevance to what we can find in humans," said Dr. Paul Volberding, director of AIDS activities at the San Francisco General Hospital. "Even antibodies people develop themselves to fight the disease don't work."

The researchers urged an intensified effort to identify chemical compounds that could be used to stop the virus from spreading in the body.

They added that the focus of new drug development in the fight against AIDS has switched from wiping the virus out to preventing its growth.

As he has in the past, Baltimore, who served as cochairman of an NAS task force on AIDS, called for a national effort to coordinate academic, industrial and federal efforts to fight the disease.

As it has become clear that the AIDS virus can lie dormant in the body for years -- possibly as long as a decade -- efforts to stop its progression have taken on new urgency.

Many researchers use cancer therapy as a model for AIDS treatment.

Increasingly, researchers have turned to a mixture of drugs to try to repair the damage AIDS does to the body's immune system.

Largely as a result of advances in the field of recombinant DNA technology, researchers said yesterday they are convinced that therapeutic agents will eventually be developed that can stop the virus from spreading.

"I am confident that within a matter of time we will be able to control AIDS through a combination of chemotherapeutic agents," said Dr. William Haseltine of Harvard University and the Dana-Farber Cancer Center. "The only question is whether it's five years, 10 years or 15."

The difference in time would have a dramatic impact on many of approximately 2 million Americans believed to be infected with HIV, the virus that causes AIDS. Many researchers say that as many as 80 percent of those infected will eventually develop symptoms of the disease.

With more than 40,000 reported cases of AIDS in the United States -- a figure that will approach 50,000 today when the Centers for Disease Control adds new case definitions to its list -- effective treatment is the best hope for many Americans.

So far only the antiviral drug azidothymidine (AZT) has been approved for marketing as effective for some AIDS patients.

Several other antiviral drugs, which, like AZT, attempt to disassemble the virus before it can multiply in the body, are under development.

Early results from trials of dideoxycytidine (DDC), a drug that scientists hoped would work well with less adverse effects than AZT, have been mixed.

In clinical tests on AIDS patients, DDC appears to be effective, scientists said yesterday. But it causes toxic reactions that so far have forced many patients to discontinue its use.

Researchers are looking at each minute piece of the AIDS virus -- its separate, small proteins, enzymes and genes -- to find ways to interfere with the virus' ability to multiply. Their goal is to find a safe, effective drug that can be taken for years.

"Each step in the infection process provides a therapeutic opportunity for treatment," said Haseltine. "And therapy will have to be the key."