Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London, United Kingdom.|Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London, United Kingdom.|Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London, United Kingdom.|Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London, United Kingdom.|Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London, United Kingdom.