1. Department of Epidemiology and Biostatistics, School of Public Health Imperial College London, London UK
2. School of Mathematics and Statistics, The University of Melbourne, Australia
3. Laboratoire de Mathématiques et de leurs Applications de Pau, Université de Pau et des Pays de l’Adour, Pau, France [2mm] School of Mathematical and Physical Sciences, Macquarie University, Sydney, New South Wales, Australia