1. Pathogen Molecular Biology and Biochemistry Unit, Department of Infectious and Tropical Diseases, London School of Hygiene & Tropical Medicine, Keppel Street, London WC1E 7HT, UK
2. Department of Pediatrics, University of British Columbia and B. C. Children's Hospital, British Columbia, Canada
3. Glaxo Wellcome Research and Development, Stevenage, Herts, UK