Study design and population
A case-control study was conducted to explore the effectiveness of COVID-19 vaccines against the SARS-CoV-2 Omicron variant outbreak in Lu’an City, which has the highest total area of 15,451.2 square kilometers in Anhui Province and about 4.39 million permanent residents[24]. Cases included 414 SARS-CoV-2 infected patients who were diagnosed by PCR testing between 4 April and 20 April 2022 and lived in Lu’an. Controls were selected from individuals with negative SARS-CoV-2 PCR test result who were in core close contact with cases and either lived in the same household or ate together or took the same enclosed stairways or elevators. We coded 1016 core close contacts with numbers 1 to 1016 in Excel, without any prior grouping stratification. From the 1016 codes, we sampled two controls matched to each case based on a random sampling method, all with an equal chance to obtain the final 828 controls.