1. From the Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute (M.K.), Brigham and Women’s Hospital (M.K., C.A.M., J.S., M.P.), Harvard Medical School (M.K., C.A.M., E.S.S.), and the Infection Control Unit and Division of Infectious Diseases, Massachusetts General Hospital (E.S.S.) — all in Boston.