Selecta Biosciences, a Watertown company announced last week that it had begun the first clinical trial of SEL-068, a nicotine vaccine. In the same way a flu vaccine revs up your body’s immune system to round up flu viruses, Selecta’s vaccine would train the immune system to capture nicotine molecules in the bloodstream, barring them from entering your brain. Smoking a cigarette would have none of the calming, appetite-reducing, or focus-sharpening effects that smokers seek.