Nabi Biopharmaceuticals on Tuesday said its NicVAX nicotine vaccine showed promising results in helping smokers quit smoking in a small clinical trial.

The Boca Raton, Florida-based biotechnology company said 33 percent of the smokers in the study who received the strongest dose of the vaccine quit smoking, compared with 9 percent who received a placebo. The trial was a mid-stage study, designed to test the safety and effectiveness of the drug.

The vaccine is designed to cause the immune system to produce antibodies that bind to nicotine and prevent it from entering the brain, blocking the “rush” that occurs when nicotine binds to receptors in the brain. That sensation is believed to be the main physical reason that most people cannot stop smoking.

The study involved 68 smokers, who received a series of up to four injections in varied doses. Patients were given the vaccine without supplemental treatments, behavioral support or counseling.