1. Vaccine Evaluation Unit, Health Protection Agency North West, P.O. Box 209, Clinical Sciences Building, Manchester Royal Infirmary, Manchester, United Kingdom
2. Division of Medical Microbiology, University of Liverpool, 8th Floor, Duncan Building, Daulby Street, Liverpool, United Kingdom
3. Division of Child Health, University of Liverpool, Royal Liverpool Children's NHS Trust, Alder Hey, Eaton Road, Liverpool, United Kingdom