1 Nuffield Department of Population Health, University of Oxford, Oxford OX3 7LF, UK; Nuffield Department of Surgery, University of Oxford, Oxford OX3 7LF, UK. Electronic address: alison.halliday@ndph.ox.ac.uk.
2 Medical Research Council Population Health Research Unit, University of Oxford, Oxford OX3 7LF, UK; Nuffield Department of Population Health, University of Oxford, Oxford OX3 7LF, UK.
3 Department of Neurology, University Hospital Basel, Basel, Switzerland.
4 Nuffield Department of Population Health, University of Oxford, Oxford OX3 7LF, UK.