Footnotes

• Contributors Author contributions: PKB had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: CDF, TS, QZ, AB, HM, PKB. Acquisition of data: CDF, PKB. Analysis and interpretation of data: CDF, PKB. Drafting of the manuscript: CDF, TS, QZ, AB, HM, PKB. Critical revision of the manuscript for important intellectual content: CDF, TS, QZ, AB, HM, PKB. Supervision: PKB.

• Competing interests None.

• Ethics approval Cantonal ethics committee Zurich.

• Provenance and peer review Not commissioned; internally peer reviewed.