[1] Hlth Protect Agcy, HIV STI Div, Ctr Communicable Dis Surveillance, London NW9 5EQ, England
[2] Hlth Protect Agcy, Cent Publ Hlth Lab, London NW9 5EQ, England
[3] UCL Royal Free & UCL Med Sch, Ctr Infect Dis Epidemiol, Dept Primary Care, London, England
[4] UCL Royal Free & UCL Med Sch, Ctr Infect Dis Epidemiol, Dept Populat Sci, London, England
[5] UCL Royal Free & UCL Med Sch, Ctr Infect Dis Epidemiol, Dept Sexually Transmitted Dis, London, England
[6] Univ London Imperial Coll Sci Technol & Med, Dept Infect Dis & Microbiol, London, England