1. Department of Clinical Research, London School of Hygiene and Tropical Medicine, University of London, London WC1E 7HT, United Kingdom
2. The Roslin Institute, R(D)SVS, University of Edinburgh, Roslin Biocentre, Midlothian EH25 9PS, United Kingdom
3. Medical Research Council Laboratories, Viral Diseases Programme, Fajara, Banjul, the Gambia