A proven vaccine to prevent infection by the AIDS-causing virus appears to be at least five to 10 years away, researchers at the Fourth International Conference on AIDS say.

But early efforts with primitive versions of vaccines show some promise. Through a process that is largely trial and error, researchers say they have been able to stimulate the immune system to produce antibodies and disease-fighting T-cells.None of these studies shows that the potential vaccines can ward off infection by the human immunodeficiency virus, or HIV - the only proof of success. Such dramatic evidence is far down the road, said researchers at the four-day conference, which ended Thursday.

The hope, they added, is that these antibodies and cells will bolster the body's defenses against the virus once it has invaded the body, thereby defeating it.

There have been four major accomplishments in the past year, each showing that vaccines may be safe and capable of triggering an immune response:

- Researchers have located the place on the virus, called an epitope, that alerts the body to begin its immunizing response. The epitope, discovered by Repligen Corp. of Boston and the National Cancer Institute in Bethesda, Md., could be mimicked by future vaccines.

- Disease-fighting antibodies were produced by chimpanzees injected with an HIV-like vaccine made by Dr. Jonas Salk of the Salk Institute in La Jolla, Calif. Salk, the pioneer of the polio vaccine, uses a unique approach, concentrating on a vaccine that would ward off acquired immune deficiency syndrome in those already infected rather than prevent infection by the virus itself.

Critics say Salk's study was carelessly conducted, a charge he firmly denies.

- Antibodies and protective immune system cells appeared in the blood of Dr. Daniel Zagury of the Pierre-et-Marie Curie University in Paris and volunteers in Zaire, after they injected themselves with a potential vaccine.

- A study at the National Institutes of Health, not yet complete, found that gay men inoculated with a test vaccine produce low levels of antibodies against the virus.

Despite these successes, there have been major disappointments in the first tests of actual effectiveness against the virus.

Chimpanzees given doses of an antibody that blocks the HIV in lab settings were not protected against infections, according to a study by Dr. Alfred Prince of the New York Blood Center and Jorge Eichberg of the Southwest Research Foundation in San Antonio, Texas.

Also, a group of monkeys given an experimental vaccine failed to be protected from the virus, even though their antibody count increased significantly, said Dr. Ronald Desrosiers of the New England Regional Primate Research Center in Southborough, Mass.

Although many researchers said they believe a vaccine will ultimately be made, it could take several years to design one that outsmarts the HIV.

"Yes, it's possible, but it's going to take a long time. There are no easy answers," said Dr. Linda Gritz of Applied Biotechnology in Cambridge, Mass., one of about a dozen U.S. companies that are attempting to create an AIDS vaccine.