[1] Univ Calif San Francisco, Dept Med, 550 16th St,3rd Floor, San Francisco, CA 94158 USA
[2] Stanford Univ, Sch Med, Stanford, CA 94305 USA
[3] Univ Witwatersrand, Fac Hlth Sci, Sch Publ Hlth, MRC Wits Rural Publ Hlth & Hlth Transit Res Unit, Johannesburg, South Africa
[4] Univ N Carolina, Dept Epidemiol, Gillings Sch Publ Hlth, Chapel Hill, NC 27515 USA
[5] Umea Univ, Umea Ctr Global Hlth Res, Dept Publ Hlth & Clin Med, Umea, Sweden