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Contributors AG and SS designed the study. SS, AS and KH collected the data for 2015 survey. SS and PD collected data for the 2012 survey. All the authors were involved in the production of the final manuscript.
Funding The research was supported by the National Institute for Health Research Biomedical Research Centre based at Guy's and St Thomas’ NHS Foundation Trust and King's College London.
Competing interests None declared.
Provenance and peer review Not commissioned; internally peer reviewed.
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