[1] Hlth Protect Agcy, London Reg Epidemiol Unit, London SW1W 9SZ, England
[2] UCL, Dept Infect & Populat Hlth, London NW3 2PF, England
[3] Inner North West London Primary Care Trusts, Publ Hlth Directorate, London NW1 5JD, England
[4] Hlth Protect Agcy, North East & North Cent London Hlth Protect Unit, London SW1W 9SZ, England