1 Center for Communicable Disease Dynamics, Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, MA 02115, USA. Electronic address: mlipsitc@hsph.harvard.edu.
2 Department of Microbiology and Immunology and Department of Pediatrics, University of Iowa, Iowa City, IA, USA.
3 Department of Medicine and Department of Microbiology, Brigham and Women's Hospital and Harvard Medical School, Boston, MA, USA.