[1] Univ Oxford, Nuffield Dept Med, Oxford OX1 3SY, England
[2] Univ Calif San Francisco, Dept Med, Div Expt Med, San Francisco, CA 94143 USA
[3] Univ Calif San Francisco, San Francisco Gen Hosp, Dept Med, Positive Hlth Program, San Francisco, CA 94110 USA
[4] Karolinska Inst, Ctr Infect Dis, Dept Med, S-14186 Huddinge, Sweden