1. University of Cape Town; Department of Public Health and Primary Health Care; Cape Town South Africa
2. Medical Research Council; Biostatistics Unit; Box 19070 Tygerberg Cape Town Cape Province South Africa 7505
3. University of Oxford; National Perinatal Epidemiology Unit; Old Road Campus Old Road Headington Oxford UK OX3 7LF
4. World Health Organization; Department of HIV/AIDS; 20 Avenue Appia Geneva Switzerland CH-1211