1MRC Centre for Causal Analyses in Translational Epidemiology, School of Social and Community Medicine, University of Bristol, Bristol, UK
2School of Social and Community Medicine, University of Bristol, Bristol, UK
3Departments of Pediatrics & Epidemiology, Biostatistics and Occupational Health McGill University Faculty of Medicine Montreal Canada
4Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, Boston, MA, USA