Doctors are testing a radical new way to help smokers quit: a shot that immunizes them against the nicotine rush that fuels their addiction.

The treatment keeps nicotine from reaching the brain, making smoking less pleasurable and theoretically, easier to give up. The small amount that still manages to get in helps to ease withdrawal, the main reason most quitters relapse.

If it works, the vaccine could become part of a new generation of smoking cessation treatments.

The drugs attack dependency in the brain instead of just replacing the nicotine from cigarettes in a less harmful way, like the gum, lozenges, patches and nasal sprays sold today.

One such drug, Pfizer Inc.'s Chantix, is due on the market any day now. Another, Sanofi-Aventis SA's Acomplia, recently won approval in Europe as a weight loss drug. If U.S. regulators follow suit, some doctors say they also will use it to help smokers quit, especially those concerned about gaining weight.

Other novel drugs are in development, but NicVax, by Nabi Biopharmaceuticals -- a Boca Raton, Fla., biotech company with labs in Rockville, Md. -- is most advanced among the vaccines.

The Food and Drug Administration has granted the vaccine fast-track status, meaning it will get prompt review, and the National Institute on Drug Abuse just gave Nabi a second $4-million grant to finance the study and NicVax's development.

"It's going to be a very good way to keep people from relapsing," said Dr. Frank Vocci, director of medications development at the federal institute.

Relapse is the biggest problem quitters face.

Of the more than 48 million smokers in the United States, 40% each year make a serious attempt to quit, but fewer than 5% have long-term success. Nicotine-replacement products combined with counseling can double that rate, but most quitters don't try them. Two-thirds go back to smoking within a month.