1. Department of Infectious Disease Epidemiology, St Mary's Campus, Imperial College London, London, W2 1PG, UK
2. Baylor College of Medicine, Houston, TX 77060, USA
3. Department of Epidemiology, Rollins School of Public Health, Emory University, Atlanta, GA 30322, USA
4. Centre for Paediatrics, Blizard Institute, Queen Mary University of London, London, E1 2AT, UK
5. Centre for Global Vaccine Research, Institute of Infection & Global Health, University of Liverpool, Liverpool, L69 7BE, UK