The vaccine, called NicVax, stimulates the production of antibodies that bind to nicotine in the bloodstream. The bound nicotine molecules are too large to enter the brain, thereby subverting the rewarding effects of the drug. Preliminary studies have shown that smokers who achieved the highest level of nicotine antibodies had higher rates of quitting smoking and longer durations of abstinence than those given placebos. The vaccine also had few side effects. Some evidence suggests that the antibodies may persist for only six to 12 months, but that may be enough to allow smokers to get through the difficult first months of withdrawal. Smoking is the largest cause of preventable deaths in the United States, with more than 400,000 deaths directly linked to it each year. 