Department of Public Health, Section of Epidemiology, University of Copenhagen, Faculty of Health and Medical Sciences, Copenhagen K, Denmark

Correspondence to Mads Kamper-Jørgensen, Department of Public Health, Section of Epidemiology, University of Copenhagen, Faculty of Health and Medical Sciences, Øster Farimagsgade 5, Post Box 2099, DK-1014, Copenhagen K, Denmark. Tel: +45 35327391; e-mail: [email protected]