1. Botswana Harvard AIDS Institute Partnership for HIV Research and Education, Gaborone 999106, Botswana
2. School of Allied Health Professions, Faculty of Health Sciences, University of Botswana, Gaborone 999106, Botswana
3. Department of Biological Sciences, Faculty of Science, University of Botswana, Gaborone 999106, Botswana
4. Department of Immunology and Infectious Diseases, Harvard School of Public Health, Boston, MA 02115, USA