Received: 23 April 2020
Accepted: 23 October 2020
First Online: 16 November 2020
Ethics approval and consent to participate
: Written informed consent was obtained from the children, their parents, and/or the schoolmaster and teachers after a complete explanation of this study before participating in the study. The study was approved by the ethics committee of the Hokkaido University.
: None applicable.
: The authors declare that they have no competing interests.