1. Futures Group/Constella, Glastonbury, CT 06033, USA.
2. National Institute of Public Health (INSP), Cuernavaca 62508, and Centro de Investigación y Docencia Económicas (CIDE), Mexico City 01210, Mexico.
3. United Nations Children's Fund (UNICEF), New York, NY 10017, USA.
4. Joint United Nations Programme on HIV/AIDS (UNAIDS), Geneva CH-1211, Switzerland.
5. Imperial College, London SW7 2AZ, UK.