Biogen Idec Inc. led gains in health care stocks, surging 6.4 percent. In a small, early stage trial, Biogen’s drug BIIB037 reduced beta amyloid in the brain and significantly improved cognition, the company’s research chief said at a Deutsche Bank conference in Boston Tuesday. Beta amyloid, a protein fragment that creates plaque tangles in the brain, is thought to be a key component in Alzheimer’s disease. The company is planning a late-stage trial of the treatment.