1. Health Protection Agency, Centre for Emergency Preparedness and Response, Salisbury SP4 0JG
2. Vaccine Evaluation Unit, Health Protection Agency North West, Manchester Royal Infirmary, Manchester M13 9WZ
3. Health Protection Agency, Centre for Infections, Colindale, London NW9 5EQ
4. School Of Medicine & Biomedical Sciences, Medical School, University of Sheffield, Sheffield S10 2RX, United Kingdom