Received: 9 December 2020
Accepted: 5 April 2021
First Online: 8 May 2021
: Our study was performed in accordance with the guidelines outlined in the Declaration of Helsinki and approved by the Research Ethics Committee of Guangzhou CDC. Informed consent was waived by the Research Ethics Committee of Guangzhou CDC, since we analyzed the data anonymously.
: Not applicable.
: The authors have declared no conflicts of interest.