Competing interests: None declared. Ethical approval: Ethics committee of Côte d’Ivoire’s Ministry of Public Health and the Institutional Review Board of the US Centers for Disease Control and Prevention

Lucien, Bondo Monga, Delphine Chia N’guessan, Déborah N’Guessan-Yao, Jean Louis N’Jampo, Patricia Patindé, Albert Seri Sekou, Odette Tossou, Leo Weakland, Christiane Wondji Gozo. At the US Centers for Disease Control and Prevention, we thank Eliane Dogoré for help with coding; David Fluker, Eleanor McLellan, and Robert Strotman for software support; and Thomas Peterman, Marc Bulterys, Richard Jenkins, RJ Simonds, and Monica Nolan for commenting on earlier versions of the manuscript. We also thank two BMJ reviewers for their useful comments and suggestions. This study was presented in part at the XIIth International Conference on AIDS and STDs in Africa, Ouagadougou, Burkina Faso, December 9-13, 2001 [abstract 10BT2-6]. Contributors: See bmj.com. Funding: The research project was funded by the Centers for Disease Control and Prevention (CDC). All of the authors were employed directly or indirectly by the CDC during the design, collection and analysis of data, and write-up of findings. Competing interests: None declared. Ethical approval: Ethics committee of Côte d’Ivoire’s Ministry of Public Health and the Institutional Review Board of the US Centers for Disease Control and Prevention.