Thirty-eight percent of participants achieved the desired level of antibodies or higher. This group also had more "clean" urine samples than those with lower antibody levels and those in the placebo group (45 percent versus 35 percent).
Higher levels were first seen at week eight and then dropped off precipitously between weeks 16 and 24, the researchers found.
Side effects were mainly restricted to tenderness around the injection site.
There is some difficulty translating these findings into a "real-world" setting, experts said.
"These cocaine users were also methadone-maintained and opiate-dependent, representing a small subgroup of those addicted to cocaine," Parsons noted. "From this study, we have no idea whether or not this vaccine will be effective in those whose primary, or even sole addiction, is to cocaine."
And the participants were already in a methadone-maintenance program, giving them more incentive to stay in the trial, Prenzlauer said.
The study was sponsored by the U.S. National Institute on Drug Abuse and the Veterans Affairs Mental Illness Research, Education and Clinical Center. Martell is also a medical director at drug company Pfizer.
There's more on cocaine and other substance abuse at the U.S. National Institute on Drug Abuse.
SOURCES: Jeffrey T. Parsons, Ph.D., professor and chair of psychology, Hunter College, New York City; Steven Prenzlauer, M.D., assistant professor of psychiatry and behavioral science, Texas A&M Health Science Center College of Medicine, Round Rock, and director of psychiatry and behavioral health, Lone Star Circle of Care, Round Rock, Texas; Jean Bidlack, Ph.D., professor of pharmacology and physiology, University of Rochester Medical Center, Rochester, N.Y.; Nora Volkow, M.D., director, U.S. National Institute on Drug Abuse; Thomas Kosten, M.D., professor of psychiatry and neuroscience, Baylor College of Medicine, Houston; October 2009, Archives of General Psychiatry