Definitions
A case was defined as an individual who tested positive in a medical institution regardless of the presence of symptoms and had polymerase chain reaction (PCR)- confirmed COVID-19 testing performed in the Lu’an Center for Disease Control and Prevention (LACDC), in which all testing organizations followed the same diagnostic criteria. An asymptomatic infection was defined as a PCR-confirmed individual who didn not meet any of the following clinical criteria: fever, cough, sore throat, or other self-perceived or clinical-identifiable symptoms or signs, and had no radiographic evidence of pneumonia. An infected patient was defined as an individual who tested positive no matter whether he or she was symptomatic. Close contact was defined as an individual without effective protection who was in contact with someone with COVID-19 within 4 days of the time before the infected person developed symptoms or the sampling date if he or she did not have symptoms. A core close contact was defined as an individual who was in frequent close contact (such as living, eating, or working in the same room, sharing meals and enclosed hallways, taking the elevator together, or attending social activities without wearing a mask, or maintaining a high risk of exposure) with an infected patient without effective protection within 2 days of the onset of symptoms, or within 2 days prior to testing of an asymptomatic patient.