1. Faculty of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, Keppel St., London WC1E 7HT, UK
2. Centre for Vaccine Development, Avenue Mohamed VI, Djikoni, 251, Bamako, Mali
3. Department of Immunization, Vaccines and Biologicals, World Health Organization, 20 Avenue Appia, 1211 Geneva 27, Switzerland