[1] Univ London London Sch Hyg & Trop Med, Hlth Serv, Res Unit, London WC1E 7HT, England
[2] Univ London Imperial Coll Sci Technol & Med, Dept Med, London SW7 2AZ, England
[3] MRC Biostat Unit, Cambridge, England
[4] MRC Biostat Unit, Cambridge, England
[5] Hlth Protect Agcy, Stat Modelling & Bioinformat Dept, London, England
[6] Univ Southampton, Div Infect Inflammat & Repair, Southampton, Hants, England
[7] Med Sch, Sch Clin Med Sci, Newcastle Upon Tyne, Tyne & Wear, England