Scientists say they have developed a cocaine vaccine that can prevent addicts from getting high by blocking the drug's effect on the brain, though it does not blunt cravings for the drug. Although the vaccine had only limited success, it created enough of an immune response in some subjects that many of them cut their drug use by half, researchers reported. The vaccine stimulates production of antibodies that bind to cocaine and hold it in the bloodstream, preventing it from reaching the brain and other organs. The trial's success is significant because some of the same principles are being used by other scientists in trying to create an antinicotine vaccine, which could have even broader appeal. 'This is the first study that has ever been done with an illicit drug to show that a vaccine can be effective in humans,' said Dr. Thomas R. Kosten, professor of psychiatry and neuroscience at Baylor College of Medicine in Houston and principal investigator of the study, published in Tuesday's Archives of General Psychiatry. An earlier, separate pilot study found similar results. 'This is establishing the principle for all drugs of abuse, whether it's nicotine or heroin or methamphetamines,' Kosten said. 'We've made vaccine for all of those things in animals, and we can put them in humans.' Several experts suggested that the vaccine might be most effective in preventing relapse in former cocaine users. Addiction experts not involved with the research said that it was an interesting first step, but that the vaccine had yet to be proven an effective treatment and needed to be improved upon. About 1.4 million Americans abuse cocaine, but there are no Food and Drug Administration-approved drug therapies to reduce dependence. The vaccine was created by linking cocaine molecules to a cholera toxin. But although 55 of the participants in the vaccine group received all five shots, only 21, or 38 percent, developed enough antibodies to block the effect of cocaine on their brains. Those who reached the threshold had significantly more cocaine-free urine samples than the others between the 9th and 16th weeks of the six-month study, when the vaccine was most effective, and 53 percent of them reduced their cocaine use by half, compared with only 23 percent in the group that was vaccinated but did not make sufficient antibodies. 'That was pretty impressive, because these were heavy daily users of cocaine, and in effect, they mostly stopped using cocaine, though they tested it every couple weeks, to see, 'Gee, can I get high now?'' Kosten said. 