Footnotes

• The contents and views in this manuscript are those of the authors and should not be construed to represent the views of the National Institutes of Health or any of the sponsoring organisations and agencies of the US government.

• Contributors DJM was involved in the design of the study, participated in the data collection and analysis, and wrote the manuscript. He approved the final version to be published and agrees to be accountable for all aspects of the work. RM was involved in the design of the study, served as an advisor in the data analysis, and revised the manuscript. He approved the final version to be published and agrees to be accountable for all aspects of the work. IS was involved in the design of the study, participated in the data collection and analysis, and revised the manuscript. He approved the final version to be published and agrees to be accountable for all aspects of the work. JT was involved in the design of the study, served as an advisor in the data analysis, and revised the manuscript. He approved the final version to be published and agrees to be accountable for all aspects of the work. EJP-S was involved in the design of the study, served as an advisor in the data analysis, and revised the manuscript. He approved the final version to be published and agrees to be accountable for all aspects of the work.

• Funding This work was supported by the University of California San Francisco ‘Resource Allocation Program for Trainees’. This research received did not receive any additional support from any funding agency in the public, commercial or not-for-profit sectors.

• Competing interests None declared.

• Provenance and peer review Commissioned; internally peer reviewed.