1. Institute of Infectious Diseases and Molecular Medicine, Department of Pathology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa
2. Department of Statistical Sciences, Faculty of Science, University of Cape Town, Cape Town, South Africa
3. Centre for the AIDS Programme of Research in South Africa, University of KwaZulu-Natal, Durban, South Africa
4. Department of Epidemiology, Columbia University, New York, New York, USA
5. National Health Laboratory Service, Johannesburg, South Africa