MRC Centre for Outbreak Analysis and Modelling, Department of Infectious Disease Epidemiology, Imperial College, Norfolk Place, London W2 1PG, UK. t.jombart@imperial.ac.uk|MRC Centre for Outbreak Analysis and Modelling, Department of Infectious Disease Epidemiology, Imperial College, Norfolk Place, London W2 1PG, UK. t.jombart@imperial.ac.uk