Received: 3 February 2019
Accepted: 27 May 2019
First Online: 30 May 2019
Ethics approval and consent to participate
: This study was reviewed and approved by the Institutional Review Board (IRB) of the Hebei CDC. Written informed consent was obtained from each participant for personal information and blood sample collection. The study was performed in accordance with the ethical standards established in the 1964 Declaration of Helsinki and its later amendments.
: Not applicable.
: The authors declare that they have no competing interests.