MRC Centre for Outbreak Control and Analysis, Department of Infectious Disease Epidemiology, Imperial College London, London, United Kingdom. d.hollingsworth@imperial.ac.uk|MRC Centre for Outbreak Control and Analysis, Department of Infectious Disease Epidemiology, Imperial College London, London, United Kingdom. d.hollingsworth@imperial.ac.uk|MRC Centre for Outbreak Control and Analysis, Department of Infectious Disease Epidemiology, Imperial College London, London, United Kingdom. d.hollingsworth@imperial.ac.uk|MRC Centre for Outbreak Control and Analysis, Department of Infectious Disease Epidemiology, Imperial College London, London, United Kingdom. d.hollingsworth@imperial.ac.uk