Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts, United States of America , Center for Communicable Disease Dynamics, Harvard School of Public Health, Boston, Massachusetts, United States of America.|Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts, United States of America , Center for Communicable Disease Dynamics, Harvard School of Public Health, Boston, Massachusetts, United States of America.|Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts, United States of America , Center for Communicable Disease Dynamics, Harvard School of Public Health, Boston, Massachusetts, United States of America.