1. 1The Wellcome Trust Sanger Institute, Cambridge CB10 1SA, UK
2. 2Pfizer Vaccine Research and Development, Pearl River, New York, USA
3. 3London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK
4. 4School of Medicine, University of St Andrews, St Andrews, Fife KY16 9TF, UK