1. Division of Medical Virology, University of Cape Town, Observatory, Cape Town, South Africa 7925
2. National Institute for Communicable Diseases, Sandringham, Johannesburg, South Africa 2131
3. AlphaVax, Research Triangle Park, North Carolina 27709-0307
4. UNC Centre for AIDS Research, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7295
5. UNC Centre for AIDS Research, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7295 and Permanent address: Department of Microbiology, School of Medicine, Seville, Spain
6. Medical Research Council, Congella, South Africa 4013
7. Perinatal HIV Research Unit, University of Witwatersrand, Witwatersrand, South Africa 2013
8. Department of Microbiology and Immunology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7295
9. University of Natal, Durban, South Africa 4041