1. a HIV Research Branch, San Francisco Department of Public Health , San Francisco , USA
2. b Division of HIV/AIDS Prevention-Surveillance and Epidemiology, National Center for HIV, STD, and TB Preventions , Centers for Disease Control and Prevention , Atlanta , Georgia , USA
3. c Primary Care Research Center , San Francisco General Hospital , San Francisco , USA
4. d Department of Epidemiology and Biostatistics , University of California San Francisco , San Francisco , USA
5. e Center for AIDS Prevention Studies , University of California , San Francisco , USA
6. f AIDS Program , University of California , at San Francisco, San Francisco , California , USA