1. Health Protection Agency Centre for Infections, London and MRC Biostatistics Unit, Institute of Public Health, Cambridge, UK,
2. MRC Biostatistics Unit, Institute of Public Health, Cambridge, UK
3. Department of Community Based Medicine, University of Bristol, Bristol, UK
4. Department of Social Medicine, University of Bristol, Bristol, UK
5. Health Protection Agency Centre for Infections, London and Centre for Research on Drugs & Health Behaviour, London School of Hygiene & Tropical Medicine, London, UK
6. Health Protection Agency Centre for Infections, London, UK