Funding for this research was provided by:
Hamburg Süd ‘COLUMBUS’ Shipmanagement
Received: 11 April 2017
Accepted: 11 January 2018
First Online: 24 January 2018
Ethics approval and consent to participate
: The study protocol was approved by the Ethics committee of Medical Association Hamburg. All participants gave written informed consent.
: Not applicable.
: The authors declare that they have no competing interests.
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