1. Harvard AIDS Institute and Department of Immunology and Infectious Diseases, Harvard School of Public Health, Boston, Massachusetts 02115
2. Department of Microbiology and Immunology, Muhimbili Medical Centre, Dar es Salaam, Tanzania
3. Department of Community Health, Muhimbili Medical Centre, Dar es Salaam, Tanzania
4. Department of Nutrition and Epidemiology, Harvard School of Public Health, Boston, Massachusetts 02215