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.