In a study, more than twice as many people given five of the shots stopped smoking than those given fewer or phony shots, about 15 percent versus 6 percent after one year.
That is comparable to some other smoking cessation aids currently sold and could be an important new tool for people who have failed to quit on other methods, doctors said.
The results, presented Wednesday at an American Heart Association conference, do not prove the new approach works but encouraged some experts.
"It clearly shows promise" and merits a definitive study, said Dr. Frank Vocci, director of medications development at the National Institute on Drug Abuse, which has given $8 million for the research so far.
"There's merit in it," but it will not be available tomorrow, said the lead researcher, Dr. Stephen Rennard of the University of Nebraska.
The study tested NicVAX, a vaccine designed to "immunize" smokers against the rush fueling their addiction. It is made by Nabi Biopharmaceuticals of Boca Raton, Florida.
The treatment keeps nicotine from reaching the brain, taking the fun out of smoking and hopefully making it easier to give up. Some nicotine still gets in, possibly easing withdrawal, the main reason quitters relapse.
This approach, attacking dependency in the brain, is different than just replacing nicotine, as the gum, lozenges, patches and nasal sprays now sold do.
The study involved 301 longtime smokers in Minneapolis, Omaha, San Francisco, Los Angeles, Boston, New York City and Madison, Wisconsin.
Participants were given four or five shots within six months, at one of two doses, or dummy shots. Neither they nor their doctors knew who got what.
Initial shots "prime" the immune system. Later doses make it produce antibodies, which latch onto nicotine in the bloodstream and keep it from crossing the blood-brain barrier and maintaining the addiction.
One year into the study -- six months after volunteers received the last shot -- 14 percent on the lower dose and 16 percent on the higher dose of five shots had quit. Only 6 percent of those given four shots, or the fake vaccine, were off cigarettes.
"These quit rates are comparable to what's seen in other studies for things that are considered to work," Rennard said.
More people in the vaccine groups dropped out of the study -- 74 out of 201 versus 33 of the 100 in the placebo group.
Two vaccine recipients had minor side effects, Rennard said.
"These are impressive preliminary data," said Dr. Sidney C. Smith Jr., a cardiologist at the University of North Carolina at Chapel Hill and past heart association president.
Getting people to quit smoking "may well be at the top of the list" for improving public health, said Smith. Worldwide, an estimated 1.3 billion people smoke, according to the heart association and it's a leading cause of cancer and heart disease.
Others were not as impressed.
"I'm a little underwhelmed," said Dr. Timothy Gardner, a heart association spokesman and cardiologist at Christiana Care Health System in Newark, Del. "I would think we could expect better" with such a novel approach, and it is hard to understand why five shots worked and four did not, he said.
The Food and Drug Administration has granted the vaccine fast-track status, meaning it will get prompt review.
Two similar vaccines are in midstage testing: TA-Nic, by Bermuda-based Celtic Pharmaceuticals, and NicQb, a product whose marketing rights Cytos Biotechnology AG recently sold to Switzerzland-based Novartis AG.
A similar "brain approach" to smoking cessation is taken by Pfizer Inc.'s Chantix, a drug that went on sale in August 2006. In a study of it, researchers reported one-year smoking abstinence rates of 22 percent versus 16 percent of those given the smoking cessation drug Zyban.
With the vaccine, people who have not quit may require periodic boosters to keep trying, Vocci said.
Of the roughly 46 million smokers in the United States, 40 percent each year make a serious attempt to quit, but fewer than 5 percent succeed long-term.
Mario Musachia had tried many times before joining the NicVAX study at the University of Wisconsin-Madison. He still struggles with the addiction, though it is not yet known whether he received vaccine or dummy shots, said Douglas Jorenby, the psychologist who heads that study site.
"We definitely have quitters and people who are absolutely convinced they got the active vaccine," Jorenby said.
One woman did not want to be told whether she had received the real thing or fake vaccine.
"She was completely focused on the fact that she was quit and she didn't want anything to undermine that," Jorenby said.