1 Department of Health Services Research and Policy, Faculty of Public Health and Policy, London School of Hygiene and Tropical Medicine, London, England, UK. Electronic address: silvia.moler@lshtm.ac.uk.
2 Department of Health Services Research and Policy, Faculty of Public Health and Policy, London School of Hygiene and Tropical Medicine, London, England, UK.
3 Centre for Longitudinal Studies, UCL Social Research Institute, University College London, London, England, UK.