- Correspondence to Thomas Edward Cowling, Department of Primary Care and Public Health, Imperial College London, Reynolds Building, St Dunstan's Road, London W6 8RP, UK;
- Received April 28, 2015
- Revised July 14, 2015
- Accepted July 17, 2015
- First published August 25, 2015.
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- Data supplement 1 - Online supplement
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