A vaccine aimed at helping people quit smoking by blunting the effects of nicotine doubled the number who could kick the habit but overall success remained small, U.S. researchers said on Wednesday.

Nabi Biopharmaceuticals (NABI.O: Quote, Profile, Research) said 16 percent of volunteers vaccinated with the highest dose were able to quit smoking for 12 months, compared with 6 percent of those who got a placebo shot.

Fourteen percent who got a lower dose stopped smoking for at least a year, compared with 6 percent on the placebo, said Dr. Stephen Rennard of the University of Nebraska Medical Center.

Those who were unable to quit smoked fewer cigarettes, Rennard found.

"I believe data from this trial are very encouraging for smokers who are trying to quit as well as for the field of smoking cessation vaccines," Rennard said in a statement.

The vaccine is designed to make the body produce immune system chemicals called antibodies that stop nicotine from getting to the brain and producing its pleasurable effects.

Rennard said patients who produced the most anti-nicotine antibodies were also the ones most likely to stop smoking for longer. And if they did not quit, they smoked less -- 10 cigarettes on average a day, compared with 20 before they got the vaccine.

"This development is key for the field of smoking cessation research and could have a significant impact on how we treat patients with nicotine addiction," said Rennard, who presented his findings to a meeting of the American Heart Association in Orlando, Florida.

(Reporting by Maggie Fox; Editing by John O'Callaghan)