Received: 23 October 2018
Accepted: 5 June 2019
First Online: 14 June 2019
Ethics approval and consent to participate
: The study was reviewed and approved by the Regional Research Ethics Committee of Norway (REK. Sør S-0643). All participants provided written informed consent. Consent to participate was obtained from the parents of children under the age of 16 years.
: Not applicable.
: The authors declare that they have no competing interests.