1. Expanded Programme on Immunization Plus, World Health Organization, avenue Appia 20, 1211 Geneva 27, Switzerland.
2. PATH, Ferney-Voltaire, France.
3. World Health Organization, Ouagadougou, Burkina Faso.
4. World Health Organization, N’Djamena, Chad.
5. Ministry of Health, N’Djamena, Chad.