Contributors: Study design: LL, GZ and JW. Data collection: LC, MC, ZH and NX. Analysis and interpretation of the data: ZZ, KQ, TH and ML. Writing of the report: LC. Revision of the draft: LC, ZZ and GZ. Making the decision to submit the paper for publication: LL, GZ, JW, LC, MC, ZH, NX, ZZ, KQ, TH and ML.