1. AIDS Research Unit, National Institute for Virology, Sandringham 2131, Johannesburg, South Africa
2. Division of Medical Virology, University of Cape Town, Rondebosch 7701, Cape Town, South Africa
3. South African National Bioinformatics Institute, University of the Western Cape, Bellville 7535, South Africa
4. Vaccine and Prevention Trials Unit, Medical Research Council, Durban 4013, South Africa