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.