Received: 21 April 2020
Accepted: 6 July 2020
First Online: 13 July 2020
Ethics approval and consent to participate
: This study protocol was approved by the Epidemiologic Research Ethics Committee of Gunma University (no. 24–26). Because this was a retrospective observational study and the data analyzed were anonymized, informed consent from participants or their parents/guardians was obtained through an opt-out method on our hospital website in accordance with the Ethical Guidelines for Medical and Health Research Involving Human Subjects in Japan.
: Not applicable.
: The authors declare that they have no competing interest.