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.