Study participants
A total of 419 cases were confirmed inside Lu’an city from 4 April to 20 April 2022. There were 4817 close contacts, among which 1025 were core close contacts. We chose 818 core close contacts as controls based on a random sampling method. However, five study participants were excluded because they were not residents of Lu’an. Consequently, 414 test-positive cases and 828 test-negative controls were included in the final analysis. The flowchart of case and control selection is shown in Figure 1.