Funding for this research was provided by:
111 Project (B16031)
Received: 22 March 2021
Revised: 15 October 2021
Accepted: 14 January 2022
First Online: 30 January 2022
: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
: This study was approved by the institutional review boards at Stanford University (Palo Alto, USA). Permission was received from local boards of education in each region and the principals of all schools. The presented data are anonymized and risk of identification is low. The principles of the Declaration of Helsinki were followed throughout.