[1] Univ Manchester, St Marys Hosp, Inst Canc Sci, 5th Floor Res,Oxford Rd, Manchester M13 9WL, Lancs, England
[2] Univ Manchester, Inst Populat Hlth, Ctr Biostat, Manchester, Lancs, England
[3] Aberdeen Matern Hosp, Dept Obstet & Gynaecol, Aberdeen, Scotland
[4] Univ Oxford, Nuffield Dept Populat Hlth, Hlth Econ Res Ctr, Oxford, England
[5] Univ York, Dept Hlth Sci, York, N Yorkshire, England
[6] Cent Manchester Univ Hosp NHS Fdn Trust, Virol Dept, Manchester, Lancs, England