[1] San Francisco Gen Hosp, Posit Hlth Program, Dept Med, San Francisco, CA 94110 USA
[2] Univ Calif San Francisco, Dept Med, Epidemiol Prevent & Intervent Ctr, San Francisco, CA 94143 USA
[3] Univ Calif San Francisco, Dept Epidemiol & Biostat, San Francisco, CA 94143 USA