Helicobacter pylori infection prevalence declined among an urban health check-up population in Chengdu, China: a longitudinal analysis of multiple cross-sectional studies oleh Jin-Chen Zou, Jin-Chen Zou, Mao-Yao Wen, Yan Huang, Xin-Zu Chen, Xin-Zu Chen, Xin-Zu Chen, Jian-Kun Hu, on behalf of the SIGES Research Group