Nucleic acid test results
An analysis conducted by the CDC of Yangzhou, Jiangsu Province, China. followed up 14 asymptomatic cases and 23 confirmed cases of COVID-19 for six weeks. The results of virus nucleic acid test of respiratory tract specimen in the 2nd, 3rd and 4th week after exposure showed that the positive rates of asymptomatic cases were 100.00%, 66.67% and 20.00% , while in the confirmed group were 100.00%, 61.54% and 8.33% respectively. The positive rate fall to 0% in the 5th week in the confirmed group, while 0% appear in the 7th week in the asymptomatic cases. The researchers concluded that the positive rate was high in the 2nd week and decreased gradually in both groups, but it lasted longer in asymptomatic cases than in confirmed cases and the difference was statistically significant (P < 0.01) . Meanwhile, the average values of Ct of the novel coronavirus ORF1ab gene in asymptomatic cases in the 2nd, 3rd and 4th week were (31.01±3.68), (32.64±3.74) and (29.05±2.96), the N gene’s were (31.37±3.62), (32.90±3.51) and (28.40±4.19) respectively, and there showed no significant differences (P =0.23, 0.12)[24].We think the small sample size makes the research results have some limitations.
Another question we focused on was the communicable period , which was defined as the duration of a positive nucleic acid test. Hu et al. reported the communicable period ranged from 1 to 21 days (median:9.5 days, IQR: 3.5–13.0 days)[25], while Li et al. concluded it ranged from 8 to 21 days (average:10.9 days)[9]. They both believe children asymptomatic carriers have a longer communicable period. So far, no detection methods could be able to accurately determine contagious period of COVID-19. The initial RT-PCR false-negative rates were even up to 29%[26], which may be ascribed  to the viral infection “window period”, position of the sample collection and the experience of the operator and so on. Combination SARS-CoV-2 RNA test of different specimens with multiple detective methods such as a IgM-IgG combined antibody test or the next generation sequencing(NGS)may help to identify more asymptomatic carriers.
It has already been confirmed the viral load detected in the asymptomatic carriers was similar to that in the symptomatic patients[27], and many reports have confirmed that asymptomatic infections are contagious[14, 20, 28-32]. Hu et.al reported an asymptomatic carrier transmitted the virus to his family close contacts, and one of whom developed severe COVID-19 pneumonia monitored in ICU[25]. Some researchers also found that there is a risk of reoccurrence of positive result of nucleic acid test in asymptomatic cases after two negative results as well as in confirmed cases[24][33]. Understanding the implications of virus transmission in asymptomatic carriers is critical to developing containment strategies.