1. Department of Population Medicine, Cardiff University, Cardiff CF10 3XQ, UK
2. Respiratory Unit, Department of Surgical and Medical Sciences, University of Modena and Reggio Emilia, 41124 Modena, Italy
3. Department of Biostatistics and Health Informatics, Institute of Psychiatry, Psychology and Neuroscience, King’s College London, London SE5 8AF, UK
4. Academic Respiratory Unit, School of Clinical Sciences, University of Bristol, Bristol BS10 5NB, UK