Discussion
Cutting off the route of transmission is an important preventive measure
for infectious disease. However, it has been reported recently that
asymptomatic carrier can lead to person-to-person transmission in
community due to the neglect. This will also pose a huge challenge for
COVID-19 prevention and control. Therefore, the proportion of
asymptomatic infections needs to be determined timely through proper
laboratory techniques.
There are two situations in asymptomatic patients with COVID-19:
infected individuals with positive viral nucleic acid test results but
no recognizable symptoms and signs after observation for 14 days, and
asymptomatic infections within the incubation period. As shown here, the
nucleic acid test results of the infected individuals could be positive,
even without self-perceived or clinically recognizable symptoms and
signs during sampling, however, clinical manifestations could appear
later.
It was reported that a 20-year-old female asymptomatic patient infected
the five other members of her family after returning to Anyang, Henan
province of China in January 10, 2020 [6]. The
five family members developed symptoms (fever and cough) and were
diagnosed with COVID-19. Nevertheless, the asymptomatic patient remains
free of clinical symptoms, and her C-reactive protein and chest CT
examination were normal. With the emergence of increased cases from
abroad, the asymptomatic SARS-CoV-2 populations have attracted more
attention as a hidden source of infection. As of August 21, 2020, there
are 353 asymptomatic infections under medical observation in China,
including 248 imported cases [7].
Identifying asymptomatic infected populations early and accurately is
essential for the epidemic prevention and control. Here, we provide
evidence for screening infection in 26 asymptomatic individuals through
dynamic analysis of IgM and IgG antibodies. It provides a laboratory
basis for understanding the status of the immune system and the pattern
of specific antibodies production. In this study, 33 asymptomatic cases
with positive nucleic acid were screened, and 7 of them were converted
to confirmed COVID-19 cases following their development of fever, cough,
and chest CT imaging changes during the period of medical quarantine and
observation, the remaining 26 remained asymptomatic. However, these 7
patients with COVID-19 did not receive continuous quantitative
monitoring of serum antibody. So, it is impossible to compare and
analyze the characteristics of antibody between asymptomatic and
confirmed patients.
To further evaluate the production of SARS-CoV-2 specific antibodies in
asymptomatic patients, blood specimens were collected regularly to
detect the levels of serum SARS-CoV-2 specific IgM and IgG antibodies
after nucleic acid converted to negative. The positive rate of IgM
antibodies in asymptomatic individuals was significantly higher even
when their nucleic acid converted to negative than that in healthy
people. However, there was no significant difference in changes within 7
weeks after nucleic acid negative conversion (p<0.05). Our
previous study showed that the positive rate of IgM in patients with
COVID-19 was 75.9% [8]. These results suggest
that IgM may be a useful target in screening the cases who previously
infected by SARS-CoV-2 and healthy people. Furthermore, IgM in some
asymptomatic individuals after nucleic acid convert to negative is not
easy to degrade within 7 weeks.
According our study, the serum concentration of IgG in asymptomatic
individuals after the nucleic acid converted to negative were in high
level and increased with time in the 7 weeks we observed. The positive
rate of serum IgG in asymptomatic group was 93.5% during 7 weeks after
the nucleic acid turned to negative. While Long QX, et al[9] reported that the positive rate of serum IgG
in patients with acute phase COVID-19 was 18.9%, while in
convalescent-phase COVID-19 was 60.0%, patients with acute phase.
To date, studies have focused more on antibodies in patients with a
confirmed diagnosis rather than changes in antibody levels in
asymptomatic patients following the nucleic acid negative conversion.
For instance, Fei Xiang at al. found that the sensitivity and
specificity of IgM were 77.3% and 100%, respectively, and the
sensitivity and specificity of IgG were 83.3% and 95.0%, respectively,
during detection of antibodies in COVID-19 patients[10]. Our previous study found that the
sensitivity of IgG (90.5%) was higher than that of IgM (75.9%), and
the specificity of IgG (99.3%) was higher than that of IgM (94.0%) in
COVID-19 patients [8]. Taken together, this indicates that the
positive rate of IgM in asymptomatic patients after nucleic acid
negative conversion is significantly lower than that in confirmed
patients, while IgG remains at the same level as that of confirmed
patients. Unfortunately, this study was unable to obtain serum specimens
from asymptomatic patients when their nucleic acid was positive, and
thus it was not possible to compare changes in antibody levels between
nucleic acid positive and negative.
In conclusion, nucleic acid testing, though time consuming and
susceptible to sampling errors, is recommended to be the main basis for
the diagnosis of asymptomatic patients. Antibody detection holds great
value in the diagnosis and identification of asymptomatic patients as it
is fast, convenient, and the sampling is easily standardized.