1. Department of Biostatistics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA
2. Division of Infectious Diseases, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA
3. Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA
4. Department of Epidemiology, Carolina Population Center, University of North Carolina, Chapel Hill, North Carolina, USA