Funding for this research was provided by:
Bundesministerium für Bildung und Forschung (01GY1328)
Received: 2 January 2019
Accepted: 30 July 2019
First Online: 8 August 2019
Ethics approval and consent to participate
: The study was conducted in accordance with national law and the 1964 Helsinki declaration and its later amendments, and according to the recommendations of the guidelines on Good Epidemiological Practice [CitationRef removed]. Ethical approval was given by the institutional review board of the University Duisburg-Essen.
: Not applicable.
: The authors declare that they have no competing interests.