Stock in Nabi Biopharmaceuticals Inc. plunged 66.4 percent Monday on news that its smoking cessation vaccine, NicVAX, missed its primary endpoint in a Phase III trial. Participants in the double-blinded, placebo-controlled trial of 1,000 people quit smoking at a similar rate whether they were randomized to the vaccine or placebo group, about 11 percent. The company expressed surprise and disappointment, and is awaiting results of its second Phase III trial.