Discussion
In a previous analysis of 76 confirmed COVID-19 patients in China, the
number of viral copies was higher in the lower respiratory tract than in
throat swabs and in nasal swabs when the ORF1ab and Ngenes were targeted by using RT-PCR (Yu et al., 2020).
In addition, in sputum specimens, the number of viral copies was higher
in the early and progressive stages than in the recovery stage (Yu et
al., 2020).
In another study of 18 confirmed COVID-19 patients in China, PCR was
performed for the ORF1ab and N genes by using nasal and
throat swabs. A high number of viral copies was confirmed in the test
that was performed not long after the onset of symptoms, and the number
of copies was higher in the nasopharyngeal sample than in the
oropharyngeal sample. In addition, the viral load was detected in both
symptomatic and non-symptomatic patients (Zou et al., 2020).
Considering the information in the literature about reported cases of
severe acute respiratory syndrome (SARS), the viral load increased from
the early phase of the disease after symptom onset and peaked after
approximately 10 days. The viral load then gradually decreased, and
virus shedding was observed from 10 to 21 days (Peiris et al., 2003). In
another study on SARS, the viral load in nasopharyngeal swabs peaked on
day 10 after the onset of symptoms. The serum viral load showed a
proportional relationship with oxygen saturation reduction, mechanical
breathing, and death (Hung et al., 2004).
Only a few studies have compared the number of viral copies of
SARS-COV-2 during the incubation period and symptomatic phase.
Studies involving patients with Middle East respiratory syndrome (MERS)
have also shown that the mortality rate in MERS patients increased with
a high viral load (Min et al., 2016). In addition, in studies on the
viral load and severity of MERS-CoV, a lower Ct value in the sample
taken from the upper respiratory tract was associated with a higher
mortality rate and higher ICU hospitalization rate (Feikin et al.,
2015). Another study showed that the peak and mean viral loads tended to
be similar between MERS patients and SARS patients (Corman et al.,
2016).
In other respiratory viruses (e.g., respiratory syncytial virus
[RSV]), a high viral load in the upper respiratory tract is
associated with disease severity (Fuller et al., 2013).
However, in the current study, both the severe and mild groups of
patients with COVID-19 showed low Ct values at symptom onset. One
patient in the mild group even had a very low Ct value in the incubation
period. The Ct values of the lower respiratory tract specimens were also
low in the early phase after the onset of symptoms in the patients in
the mild group, exhibiting a different trend from that of patients with
other respiratory viruses such as MERS, SARS, and RSV.
In a study on SARS patients, 60 of 75 patients (80%) showed
radiological worsening after a mean of 7.4 days (Fuller et al., 2013).
In another study of 17 patients with MERS, the chest radiograph scores
peaked at approximately 2 weeks after the onset of symptoms (Oh MD et
al.,2016).
However, in the current study, the chest radiograph scores peaked in the
severe group at approximately 3 weeks after the onset of symptoms.
The viral load of COVID-19 patients was high at the onset of symptoms.
The patients in the mild group also showed a high number of virus copies
in the early stages of the disease. Even in the incubation period, very
low Ct values were observed.
Therefore, it is necessary to evaluate whether an asymptomatic infected
person can transmit the virus even in the incubation period.
Studies are currently underway to confirm the contagiousness of the
virus even during the incubation period by determining whether the virus
can be cultured during the incubation period.