1. Department of Healthcare-Associated Infection and Antimicrobial Resistance, Health Protection Agency Centre for Infections, London, UK
2. Department of Statistics, Modelling and Bioinformatics, Health Protection Agency Centre for Infections, London, UK
3. MRC Centre of Epidemiology for Child Health, UCL Institute for Child Health, London, UK
4. Paediatric Infectious Disease Unit, St George's Hospital Medical School, University of London, London, UK