COVID-19 is an acute respiratory infectious disease caused by SARS-CoV-2. It is mainly transmitted through respiratory droplets and close contact and has developed into a worldwide pandemic[10, 11]. Infected with SARS-CoV-2, most patients had mild symptoms and good prognosis, and some patients rapidly progressed to severe or critical pneumonia (14% and 5% of laboratory-confirmed patients, respectively [12]), with a significantly increased mortality rate. The clinical manifestations of COVID-19 are diverse[13, 14], and existing studies suggest that older age and male gender may be associated with higher disease severity[15–18]. Fever[15, 19], shortness of breath/dyspnea[19, 20] and gastrointestinal symptoms seems to be important risk factors for severity of COVID-19[21]. What’s more, some comorbidities, such as Hypertension, Diabetes, obesity, Metabolic Syndrome, COPD and so on, may increase severe outcome[22–31]. The Charlson Comorbidity index (CCI) score has been identified in studies as a prognostic factor for COVID-19-related death [32]. Patient age and disease burden (number and severity of conditions) are directly and significantly associated with an increased risk of unfavorable clinical outcomes [33]. In this study, we did not find significant differences in age, gender and other demographic indicators between patients with severe/critical pneumonia and those with moderate pneumonia. Statistics of the clinical symptoms of COVID-19 revealed that sputum production was more common in patients with moderate pneumonia, while headache appeared to be more pronounced in severe/critical patients. Some studies suggest that SARS-CoV-2 virus is not only confined to the respiratory tract, but may also invade the central nervous system [34–37], and further studies are needed to clarify whether central nervous system infections are more pronounced in critically ill patients[38]. In addition, we similarly found that, the prevalence of comorbidities is found to be significantly different according to disease severity: higher in the severe/critical group.
We conducted simple statistics on the laboratory indicators, and the results showed that lymphocyte count, NLR and severity of COVID-19 were related. Lymphocyte count of the severe/critical pneumonia group were significantly lower, and NLR was significantly higher. This is consistent with some previous studies. A number of studies have shown that, compared with mild cases, the absolute number of lymphocytes in severe patients is significantly reduced [39, 40], and the continuous decrease of peripheral blood lymphocyte count may be an early indicator for severe/critical patients with COVID-19. A meta-analysis showed that high NLR levels on admission were associated with severe COVID-19 and mortality[41]. In addition, factors associated with low lymphocyte count and high lactate dehydrogenase levels are important and independent risk factors for adverse clinical outcomes [42, 43]. Han et al. found that serum LDH and CRP were significantly correlated with the severity of COVID-19. And Smilowitz NR et al. also came to the conclusion that CRP was strongly associated with critical illness and mortality in COVID-19[44]. In our study, CRP elevation appeared to be higher in the severe/critical group than in the moderate group (P = 0.076). However, due to the limitation of detection methods, the CRP value below 0.5 cannot be measured and accurate numerical analysis cannot be carried out.
Chest CT is a routine examination for COVID-19. At the beginning of the epidemic, clinicians selected chest CT as an important assessment tool for COVID-19 based on their experience in managing similar diseases. It has also been proved that chest CT plays an irreplaceable role in the diagnosis, differential diagnosis, clinical classification, prognosis and therapeutic effect evaluation of COVID-19 [9, 45]. In particular, CTSS has important reference value for the assessment of disease severity and the prediction of mortality [46, 47]. In addition, several studies have described the temporal changes of chest CT, and it is believed that chest CT lesions are most obvious and CT severity score highest about 9–12 days after symptom onset[47, 48]. We also found that about 2 weeks after the onset of symptoms, the lesions on chest CT were gradually absorbed over time with reduced density. Moreover, pulmonary consolidation was evident in severe/critical patients, with higher CT severity scores. These conclusions corroborate with previous studies to some extent. It should be noted, however, that imaging abnormalities in patients may persist for long periods of time. In the chest CT reexamined 50 days after the onset of symptoms, obvious ground-glass opacities and other lesions can still be seen, and the extent of the lesions is mostly smaller than before, with significantly reduced density. At this time, the patient's condition must be assessed in combination with the patient's clinical manifestations. In addition, we also observed that in a few cases, during the absorption and dissipation of the lesion, there was a significant deterioration (expansion of the extent of the lesion, appearance of new consolidation, etc.) This also reminds us that our understanding of COVID-19 is still very limited and cannot be taken lightly prematurely, especially in severe /critical patients.