1. Department of Infectious Disease Epidemiology, London School of Hygiene and Tropical Medicine , London, UK
2. Department of Population Health, London School of Hygiene and Tropical Medicine , London, UK
3. Leverhulme Centre for Demographic Science, Nuffield College and Department of Sociology, University of Oxford , Oxford, UK
4. Interdisciplinary Centre on Population Dynamics, University of Southern Denmark , Odense, Denmark