1 Ministry of Health, Government of The Gambia, Banjul, Gambia.
2 Disease Control and Elimination Theme, Medical Research Council Unit The Gambia at the London School of Hygiene and Tropical Medicine, Banjul, Gambia.
3 Vaccines and Immunity Theme, Medical Research Council Unit The Gambia at the London School of Hygiene and Tropical Medicine, PO Box 273, Banjul, The Gambia, Banjul, Gambia Ed.Clarke@lshtm.ac.uk.