[1] Univ Cambridge, Inst Publ Hlth, Dept Publ Hlth & Primary Care, Cambridge, England
[2] Emory Univ, Carter Ctr, Atlanta, GA 30322 USA
[3] Inst Publ Hlth, Biostat Unit, MRC, Cambridge, England
[4] Inst Publ Hlth, E England Reg Epidemiol Unit, Hlth Protect Agcy, Cambridge, England