Received: 6 November 2018
Accepted: 28 June 2019
First Online: 11 July 2019
Ethics approval and consent to participate
: The protocol of the 2010–2012 CNNHS was approved by the Ethical Committee of NINH, China CDC (2013–018). Signed consent forms were obtained from the children’s parents or legal guardians before data collection.
: Not applicable.
: The authors declare that they have no competing interests.