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.