1. Division of Experimental Medicine, Department of Medicine, UCSF, San Francisco, California 94110
2. Gladstone Institute of Virology and Immunology, UCSF, San Francisco, California 94158
3. Division of VCF and Epidemiology, Department of Medicine, UCSF, San Francisco, California 94143
4. Epidemiology Branch, Division of HIV/AIDS Prevention, Centers for Disease Control and Prevention, Atlanta, Georgia 30333