1. Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
2. Department of Pathology, Microbiology, and Immunology, Vanderbilt University Medical Center, Nashville, TN 37232, USA.
3. Department of Microbiology, Faculty of Biochemistry, Biophysics, and Biotechnology, Jagiellonian University, Krakow, Poland.
4. Gilead Sciences Inc., Foster City, CA 94404, USA.
5. Division of Infectious Diseases, Department of Pediatrics and Department of Pathology, Microbiology, and Immunology, Vanderbilt University Medical Center, Nashville, TN 37232, USA.