Peter J White1, P Joanna Lewis2. 1. MRC Centre for Outbreak Analysis and Modelling and NIHR Health Protection Research Unit in Modelling Methodology, Imperial College London, London, UK; Modelling and Economics Unit, National Infection Service, Public Health England, London, UK. Electronic address: p.white@imperial.ac.uk. 2. MRC Centre for Outbreak Analysis and Modelling and NIHR Health Protection Research Unit in Modelling Methodology, Imperial College London, London, UK.