[1] Univ London Imperial Coll Sci Technol & Med, Dept Primary Care & Social Med, Ctr Res Drugs & Hlth Behav, London W6 8RP, England
[2] Hlth Protect Agcy London, Sexually Transmitted & Blood Borne Virus Lab, London, England
[3] London Sch Hyg & Trop Med, Infect Dis Epidemiol Unit, London WC1, England