[1] George Washington Univ, Sch Publ Hlth, Dept Epidemiol & Biostat, Washington, DC USA
[2] London Sch Hyg & Trop Med, Dept Clin Res, London WC1, England
[3] Elizabeth Glaser Pediat AIDS Fdn, Lusaka, Zambia
[4] London Sch Hyg & Trop Med, Dept Global Hlth & Dev, London WC1, England
[5] Minist Hlth, HIV AIDS STI Programme, Lusaka, Zambia
[6] Elizabeth Glaser Pediat AIDS Fdn, Washington, DC USA