[1] Addenbrookes Hosp, Clin Microbiol & Publ Hlth Lab, Hlth Protect Agcy, Cambridge CB2 2QW, England
[2] Univ Forvie Site, Inst Publ Hlth, E England Reg Epidemiol Unit, Cambridge CB2 2SR, England
[3] Addenbrookes Hosp, Cambridge Univ Hosp NHS Fdn Trust, Dept Dermatol, Cambridge CB2 2QQ, England
[4] Univ Cambridge, Inst Publ Hlth, Dept Publ Hlth & Primary Care, Ctr Appl Med Stat, Cambridge CB2 2SR, England