[1] Univ Calif San Diego, San Francisco Gen Hosp, Dept Med, Posit Hlth Program, San Diego, CA 92110 USA
[2] Univ Calif San Diego, Div Expt Med, San Diego, CA 92110 USA
[3] Univ Calif San Diego, Dept Epidemiol & Biostat, San Diego, CA 92110 USA
[4] Univ Calif Davis, Dept Microbiol & Immunol, Davis, CA 95616 USA