Funding for this research was provided by:
European Centre for Disease Prevention and Control
Received: 21 January 2020
Accepted: 26 June 2020
First Online: 7 July 2020
Ethics approval and consent to participate
: Ethics approval was received from the local ethics committee in Stara Zagora, Bulgaria. Written informed consent was collected from all participants prior to participation in the study.
: Not applicable.
: The authors declare that they have no competing interests.