For 600 days and counting, monkeys given an experimental new AIDS vaccine have survived with no signs of illness despite exposure to lethal doses of virus, raising hopes that scientists may be headed at last toward an effective vaccine for people.

Several studies presented at an AIDS vaccine conference here Thursday suggest that novel combinations of genes and other immune system stimulators may be able to keep HIV at bay, even if they fail to prevent infection.

Testing in people has just begun, and no one knows if they will truly work. Even if they do, it may take a decade of fine-tuning and large-scale testing before they reach widespread use.

In the longest-running of these experiments, researchers from Harvard Medical School showed the approach can keep monkeys healthy for more than a year and a half after receiving a particularly lethal form of the virus that ordinarily kills within a few months.

"After 600 days, there is no evidence of disease, no evidence of rebounding virus," said Dr. Norman Letvin. "This is all good news."

Of the animals given the virus without vaccine protection, 87 percent have AIDS and three-quarters have died. None of the animals that were fully vaccinated have gotten sick.

Researchers have been carefully watching the vaccinated animals' health, worried that the encouraging early response might wear off over time, allowing the virus to overwhelm their immune systems.

"It's good to see that the protection is holding," said Dr. Harriet Robinson of the Yerkes Primate Research Center in Atlanta.

Robinson updated her own experiments at the meeting, reporting that monkeys getting a similar kind of vaccine are still alive and well one year after exposure to the virus.

Finding a vaccine against AIDS has been a top goal since the epidemic emerged 20 years ago, but only recently have researchers begun to seem hopeful that a vaccine is possible. One sign of that change is this week's AIDS Vaccine 2001 in Philadelphia, the first full-fledged scientific conference devoted to AIDS vaccines.

"I'm optimistic in a way that I wasn't a few years ago that the vaccine candidates we are testing today will carry forward and make a difference, both in the United States and abroad," said Dr. David Baltimore, head of the government's AIDS Vaccine Research Committee.

While many different vaccine strategies are in the works, those generating much of the excitement at the conference use what is called a prime-boost approach. The idea is to beef up the body's AIDS surveillance by giving an initial vaccination, then following with a different round of shots a few weeks later.

Dr. Margaret Johnston, assistant director for AIDS vaccines at the National Institute of Allergy and Infectious Diseases, said that 12 to 15 different prime-boost combinations are now in development.

One, developed by Merck & Co., is already in early human safety testing. It involves immunizing with genes that carry the code for proteins found in the AIDS virus, then following with a cold virus engineered to carry in more of these HIV genes.

The Harvard vaccine uses viral genes and then boosts that with a gene that produces interleukin-2, an immune system stimulant. The Yerkes version combines viral genes with a genetically engineered smallpox vaccine. Both are nearing human testing.

Unlike all other vaccines now in use to prevent diseases, none of these is intended to stop the infection from occurring. While that is still the ultimate goal of vaccine development, scientists do not know how to outsmart the virus and keep it from insinuating itself into blood cells.

Instead, these vaccines are designed to help the body mount a strong counterattack by killer cells once infection occurs, keeping virus levels low and allowing people to stay healthy for many years.