[1] Univ Calif San Francisco, Blood Syst Res Inst, San Francisco, CA 94118 USA
[2] Univ Calif San Francisco, Dept Lab Med, San Francisco, CA 94143 USA
[3] Univ Edinburgh, Ctr Infect Dis, Edinburgh, Midlothian, Scotland
[4] Natl Inst Hlth, Dept Virol, Islamabad, Pakistan