[1] Cent Publ Hlth Lab, Sexually Transmitted & Blood Borne Virus Lab, Publ Hlth Lab Serv, London NW9 5HT, England
[2] Univ Cambridge, Dept Publ Hlth & Primary Care, Inst Publ Hlth, Cambridge, England
[3] Publ Hlth Lab Serv, Ctr Communicable Dis Surveillance, London NW9 5EQ, England