1. Faculty of Epidemiology and Population Health, Department of Community Medicine, London School of Hygiene and Tropical Medicine, London, UK
2. Clinical Trial Service Unit, Nuffield Department of Population Health, University of Oxford, Oxford, UK
3. Department of Epidemiology and Public Health, University College London, London, UK
4. Faculty of Epidemiology and Population Health, Department of Community Medicine, London School of Hygiene and Tropical Medicine and Adjunct Professor of Epidemiology, UiT the Arctic University of Norway, Tromsø, Norway