1. Department of Virology, Royal Free and University College Medical School, London, UK; and Division of Retrovirology, National Institute for Biological Standards and Control, South Mimms, Herts, UK
2. Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, UK
3. Medical Research Council Laboratories, Banjul, The Gambia, West Africa
4. Medical Research Council Laboratories, Banjul, The Gambia, West Africa; and AIDS Research Centre, National Institute of Infectious Diseases, Tokyo, Japan
5. Laboratório Nacional de Saude Publica, MINSAP, Bissau, Guinea-Bissau, West Africa
6. Department of Virology, Royal Free and University College Medical School, London, UK
7. Department of Epidemiology Research, Danish Epidemiology Centre, Statens Serum Institut, Copenhagen, Denmark