[1] Human Sci Res Council, HIV AIDS STIs & TB Res Programme, Pretoria, South Africa
[2] Univ Limpopo, Dept Res & Innovat, Turfloop, South Africa
[3] Univ Porto, Porto Business Sch, Porto, Portugal
[4] Univ Porto, Ctr Econ & Finance cefUP, Porto, Portugal
[5] Univ Penn, Populat Studies Ctr, Philadelphia, PA 19104 USA
[6] Leonard Davis Inst Hlth Econ, Philadelphia, PA USA