1. Department of Epidemiology and Biostatistics, MRC-HPA Centre for Environment and Health; School of Public Health, Imperial College London; Norfolk Place; London; W2 1PG; United Kingdom
2. Department of Infectious Disease Epidemiology, MRC Centre for Outbreak Analysis and Modelling; Imperial College; London; United Kingdom
3. Department of Mathematics; Imperial College; London; United Kingdom
4. Institute for Risk Assessment; Utrecht University; Utrecht; The Netherlands
5. MRC Biostatistics Unit; Institute of Public Health; Cambridge; United Kingdom