1. UCL/MRC Centre for Medical Molecular Virology, University College London, Windeyer Building, 46 Cleveland Street, London W1T 4JF, United Kingdom
2. Division of Infection Diseases, Stanford University, Stanford, California 94305
3. Medical Research Centre Clinical Trials Unit, London NW1 2DA, United Kingdom
4. Centre for Infections, Health Protection Agency, 61 Colindale Avenue, London NW9 5EQ, United Kingdom