1. Institute of Cancer Sciences, University of Manchester, St Mary’s Hospital, Manchester, UK
2. Centre for Biostatistics, Institute of Population Health, University of Manchester, Manchester, UK
3. National Perinatal Epidemiology Unit, Nuffield Department of Population Health, University of Oxford, Oxford, UK
4. Health Economics Research Centre, Nuffield Department of Population Health, University of Oxford, Oxford, UK
5. Department of Obstetrics and Gynaecology, Aberdeen Maternity Hospital, Aberdeen, UK
6. Department of Health Sciences, University of York, York, UK
7. Virology Department, Central Manchester University Hospitals NHS Foundation Trust, Manchester, UK