Funding for this research was provided by:
Guangzhou Municipal Science and Technology Project (201707010184, 201607010332)
Guangzhou Medical Scientific Research Foundation (201102A211001, 20121A011123)
the National Science and Technology Major Project (2018ZX10715004)
Received: 1 September 2018
Accepted: 3 July 2019
First Online: 19 July 2019
Ethics approval and consent to participate
: The study protocol was approved by the Ethics Committee of Guangzhou Center for Disease Control and Prevention in January 15, 2009 (Approved No. of ethic committee: GZCDC2009003). Consents were obtained from all participants by using an online ‘click to consent’ procedure.
: Not applicable.
: The authors declare that they have no competing interests.