aDepartment of Epidemiology
bDepartment of Medicine
cDepartment of Microbiology, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA.
Correspondence to Kimberly A. Powers, PhD, MSPH, Department of Epidemiology, Gillings School of Global Public Health, The University of North Carolina at Chapel Hill, 2105D McGavran-Greenberg Hall, CB 7435, Chapel Hill, NC 27599-7435, USA. E-mail: firstname.lastname@example.org
Received 20 February, 2014
Accepted 26 February, 2014