[1] Univ Witwatersrand, Hlth Econ & Epidemiol Res Off, Sch Clin Med, Fac Hlth Sci,Dept Internal Med, 39 Empire Rd, ZA-2193 Johannesburg, South Africa
[2] Univ Witwatersrand, Sch Publ Hlth, Fac Hlth Sci, Johannesburg, South Africa
[3] London Sch Econ & Polit Sci, Dept Hlth Policy, London, England