3.1. Clinical manifestations
Based on the current epidemiological investigation, the incubation period of the COVID-19 is about 1-14 days (Linton et al., 2020). It has been reported that the incubation period for severe cases may be shorter than that of the mild cases and it tended to be shorter among patients older than 60 than those aged less than 60 years old. The most common symptoms at onset of COVID-19 illness according to WHO report that was done on more than 70,000 cases in China are as following: fever (88%), cough (68%), and fatigue (38%), dyspnea (21.9%) (Chappell, 2020; Pranab, Nazia, Anup, Bhabatosh, & Raman, 2020). Non-respiratory symptoms like nausea, vomiting, and diarrhea were uncommon presentations (Luigi Angelo, Giovanni, & Giacomo De, 2020). Infected children might appear asymptomatic or present with fever, dry cough, and fatigue, and few presented with gastrointestinal symptoms including abdominal discomfort, nausea, vomiting, abdominal pain, and diarrhea (K. Shen et al., 2020). According to the report on “Diamond Princess”, 189 (17.9%) patients were asymptomatic carriers (Lee, 2020), which indicated that there exist a large number of asymptomatic individuals remain undiscovered in the community. It is crucial to identify and isolate asymptomatic carriers and mild patients to contain the outbreaks in later stages.
According to the severity of symptoms, COVID-19 can be divided into four classes: mild, moderate, severe, and critical types (Zu et al., 2020). Mild patients presented low-grade fever, mild fatigue or dry cough only. Moderate patients are always characterized by fever, respiratory symptoms, and radiographic abnormality. The severe patients should meet one of these criteria: A. dyspnea B. PaO2/FiO2 <300 mmHg. C. Oxygen saturation < 93% in ambient air (Z. Sun, Thilakavathy, Kumar, He, & Liu, 2020). And about one-third of the severe patients may develop into critically ill with acute respiratory distress syndrome, septic shock, and coagulation dysfunction within a week (Gorbalenya, 2020). According to the recent survey conducted by the China CDC, 80.9% of confirmed patients were mild pneumonia cases, 13.8% were severe cases, and 4.7% were the critical cases (Surveillances, 2020).
3.2Laboratory examination
In the early stage of the COVID-19, the white blood cell count is normal or decreased, with decreased lymphocyte count. The lymphocyte subtypes were further analyzed and found that the count of CD3+, CD4+ and CD8+ T cell in the severe patients was significantly lower than that of the non-severe group (Xiaobo et al., 2020). Some patients may present elevated levels of liver enzymes (TBIL DBIL ALB ALT AST), lactate dehydrogenase, C-reactive protein (CRP), and coagulation disorders (D-dimer, fibrinogen) (D. Wang et al., 2020). The level of cardiac troponin I, N-terminal pro-brain natriuretic peptide and creatinine may be increased in severe and critically ill patients, which indicated that COVID-19 could induce myocardium, kidneys, and liver injury directly or indirectly (Jin et al., 2020). Critical patients often have a higher level of inflammatory factors such as IL2, IL6, IL7, IL10, and TNF-α (Chaolin et al., 2020). Chen et al. reported that elevated levels of C-reactive protein, D-dimer, and procalcitonin were associated with severe patients compared to non-severe patients (C. Huang et al., 2020). Zhao et al. found that a higher level of D-dimer, high-sensitivity cardiac troponin I, serum ferritin, lactate dehydrogenase, and IL-6 were the independent risk factors of death (Chaomin, 2020).
Nucleic acid testing is the standard method for the diagnosis of COVID-19 infections (Udugama et al., 2020). Based upon the evidence from clinical research, the SARS-CoV-2 nucleic acids can be detected in nasopharyngeal swabs, sputum, lower respiratory tract secretions using reverse transcription-polymerase chain reaction (RT-PCR) method (Rothan & Byrareddy, 2020). RT-PCR has been widely deployed in diagnostic virology and has yielded few false-positive outcomes. However, some research suggested that this method usually showed false-negative results due to poor RNA stability, the quality of the kit and specimen (Kimberly E. Hanson, 2020; Udugama et al., 2020). Both IgM and IgG titers were markedly increased in nearly all patients, which was normally considered as a transition from earlier to later period of infection (Zhang et al., 2020). Therefore, the combination of RT-qPCR and IgM/IgG antibody assay may be useful for diagnosing COVID-19 and screening of SARS-CoV-2 carriers, symptomatic or asymptomatic (Imai et al.; Z. Li et al., 2020).