1. a Department of Epidemiology and Biostatistics, Arnold School of Public Health , University of South Carolina , Columbia , USA
2. b Division of HIV/AIDS Prevention , Centers for Disease Control and Prevention , Atlanta , USA
3. c Department of Health and Environmental Control, HIV/STD Division , Columbia , USA
4. d Division of Infectious Diseases, Department of Medicine , University of South Carolina School of Medicine , Two Medical Park, Columbia , USA