But the researchers say the addicts would need repeat injections.
The vaccine, called TA-CD could be dangerous. Some of the addicts participating in a study of the vaccine started doing massive amounts of cocaine in hopes of overcoming its effects, according to Thomas R. Kosten, the lead researcher on the study, which was published in the Archives of General Psychiatry in October.
"After the vaccine, doing cocaine was a very disappointing experience for them," said Kosten, a professor of psychiatry and neuroscience at Baylor College of Medicine in Houston.
Nobody overdosed, but some of them had 10 times more cocaine coursing through their systems than researchers had encountered before, according to Kosten. He said some of the addicts reported to researchers that they had gone broke buying cocaine from multiple drug dealers, hoping to find a variety that would get them high.
Of the 115 addicts in the study, 58 were given the vaccine, administered in a series of five shots over 12 weeks, while 57 received placebo injections. Six people dropped out before the end of the study. The researchers recruited the participants from a methadone-treatment program in West Haven, Conn., which made it possible to track them for the full 24 weeks of the study. The patients were addicted to cocaine and heroin; TA-CD is designed to work only on cocaine, including the crack form of the drug.
TA-CD is supposed stimulates a person's immune system to produce antibodies. Of those who received all five vaccine injections, 38 percent reached antibody levels that were high enough to dull the effects of the drug. The antibodies stayed active for eight to 10 weeks after the last shot.
"In this study, immunization did not achieve complete abstinence from cocaine use," Kosten said. "Previous research has shown, however, that a reduction in use is associated with a significant improvement in cocaine abusers' social functioning and thus is therapeutically meaningful."
About a quarter of those who received the vaccine did not make sufficient antibodies at all; Kosten isn't sure why.
"That's the million-dollar question," said Margaret Haney, a professor of clinical neuroscience at Columbia University Medical Center, who is also researching the cocaine vaccine though she was not involved in Kosten's study.
In October, the journal Biological Psychiatry published online an article by Haney that also tested the effects of TA-CD.
She and her colleagues gave crack cocaine to each man 39 times over 13 weeks while monitoring his heart. ("A nurse held a flame on the cocaine and participants were instructed to take one large inhalation and hold it as long as they would outside the laboratory," according to the study.) The researchers vaccinated each participant with TA-CD on weeks 1, 3, 5 and 9 and periodically asked him to fill out a survey about his mood.
In the conclusion of her study, Haney suggested that the vaccine could help protect motivated treatment-seekers from relapse because if they slipped and used some cocaine, they wouldn't get high and trigger the craving for more drugs.
Regarding the ethics of giving laboratory-produced crack cocaine to the men, none of whom was seeking treatment for his addiction at the time of the study, Haney said that the benefits of developing a vaccine outweighed any potential harm. She said scientists have been doing these types of studies -- funded by the federal government -- for 20 years under safe, controlled conditions. "I sleep well at night because it's unethical not to do well-designed studies," she said.
A larger six-site clinical trial of the vaccine organized by Kosten is scheduled to start in the spring.
The idea of developing a medication to block addiction has long been attractive: Disulfiram (now sold under the name Antabuse), which makes people ill if they drink alcohol, has been available for alcoholism since 1948. Kosten said he hasn't received any inquiries from drug companies wanting to manufacture a large-scale cocaine vaccine. (He and Haney are conducting research on TA-CD under an agreement with the private equity firm that controls the prospective vaccine; both of their studies were largely funded by the National Institutes of Health.)
According to the 2007 National Survey on Drug Use and Health, about 2.1 million Americans had used cocaine within the previous month.
Haney said she receives phone calls from desperate people asking where they can get the vaccine for a family member who is addicted.
"They have a mistaken view of how a vaccine might work, thinking of it as magic, where what it's doing, at best, is blunting the effects," Haney said. "They get very excited, and it's heartbreaking."