Results
Epidemiology of close contacts
Among the 1,844 close contact cases, the nucleic acid swabs for 33 individuals with no clinical symptoms came back positive; these individuals were treated as asymptomatic infected patients and placed under medical observation. One week later, 7 of them developed fever, cough, and other symptoms (Figure 1, table 1), confirming they had contracted COVID-19.
The rest 26 remaining cases with positive of nucleic acid were defined as asymptomatic cases, whose average age was 33.3 ± 18.9 years old, ranging from 3-77 years old. Age between the 15 males, averaging 34.2±17.6 years old, and 11 females, averaging 32.1±21.4 years old, there was no significant difference between genders (p>0.05). Furthermore, there was no significant in age between 26 asymptomatic cases and the 7 patients with COVID-19 which was 29.7±17.5 years old (p>0.05).
Time interval from nucleic acid in positive initially to the nucleic acid in negative for the first time was defined as the communicable period, which ranged from 1-31 days, and the median communicable period (MCP) was 7.5 days.
Qualitative detection of antibodies to identify asymptomatic infections
Blood was drawn from 26 asymptomatic infected cases for nucleic acid tests with positive results. 23 of the 26 asymptomatic patients were positive for COVID-19 specific antibodies by the colloidal gold method, only 3 were negative. The total positive rate of the colloidal gold method was 88.5% (23/26), which was significantly higher than that of the healthy control group (2.0%) (χ2=59.182, p<0.001). The positive rate between the male (86.7%) and female (90.9%) antibody in asymptomatic infection had no difference significantly (p>0.05).