MARILYNN MARCHIONEFresh results from the world's first successful test of an experimental AIDS vaccine confirm that it is only marginally effective. Yet, the findings are exciting to scientists, who think they may show how to make a better vaccine. The results also hint that the vaccine may work better in the general population than in those at higher risk of infection, such as gay men and intravenous drug users. It was the first time an AIDS vaccine was tested mostly in heterosexuals at average risk, and doctors have long known that how a person is exposed to HIV affects the odds of becoming infected. "This study becomes a landmark. You can put it on a map and begin to figure out where you go from here," said Col. Jerome Kim, the U.S. Army doctor who co-led the trial. Last month, researchers announced that a two-vaccine combination cut the risk of becoming infected with HIV by more than 31 percent in a trial of more than 16,000 volunteers in Thailand. Full results, published online Tuesday by the New England Journal of Medicine and presented at a scientific conference in Paris, include two additional analyses that merely suggest the vaccine is beneficial, rather than providing definitive proof.