1. Division of Research, Kaiser Permanente Northern California , 2000 Broadway, Oakland, CA 94612, USA
2. Division of HIV, Infectious Diseases, and Global Medicine, University of California , 1001 Potrero Avenue , San Francisco, CA 94110, USA
3. Center for AIDS Prevention Studies, University of California , 550 16th Street , San Francisco, CA 94158, USA
4. Division of Biostatistics, School of Public Health, University of California , 2121 Berkeley Way , Berkeley, CA 94720, USA