Effectiveness of Inactivated COVID-19 Vaccines against COVID-19 Caused by the SARS-CoV-2 Delta and Omicron Variants: A Retrospective Cohort Study oleh Qiaoli Hua, Danwen Zheng, Bo Yu, Xinghua Tan, Qiumin Chen, Longde Wang, Jing Zhang, Yuntao Liu, Heng Weng, Yihang Cai, Xiaohua Xu, Bing Feng, Guangjuan Zheng, Banghan Ding, Jianwen Guo, Zhongde Zhang