Funding for this research was provided by:
the National Natural Science Foundation of China (81271900)
the Science and Technology Project of the Shanxi Province Health and Family Planning Commission (2011077)
Text and Data Mining valid from 2018-05-02
Received: 18 August 2017
Accepted: 20 March 2018
First Online: 2 May 2018
Ethics approval and consent to participate
: This research was carried out according to the principles of the Declaration of Helsinki and was approved by the Ethics Committees of the National Institute for Communicable Disease Control and Prevention and the Chinese Center for Disease Control and Prevention (No.: ICDC-2014005). No animal work was carried out as part of this study.
: Not applicable
: The authors declare that they have no competing interests.