An estimated 2.3% of COVID–19 patients require tracheal intubation currently [12]. A prominent complication of
advanced COVID–19 is acute hypoxemic respiratory failure requiring oxygen and MV. It is estimated that 15–20% of
infected people develop severe pneumonia and 5%–10% require critical care [12]. A recent report according to the
Chinese classification showed that 14% of patients were of the severe type and 5% were of the critical type [13].
Severe illness of COVID–19 usually begins approximately 1 week after the onset of symptoms [2], while patients have
variable phases of symptoms and disease onset at admission [12]. In the present study, most patients were classified
as having mild COVID–19 at presentation [10], and no patient received MV or high-flow oxygen therapy on admission.
In the beginning, only three patients were classified into the desaturation group. Patients in this cohort were
admitted an average of 6.8 days from symptom onset, and 12.5% were asymptomatic on admission. Nevertheless, 23.5% of
these mild patients proceeded to oxygen desaturation during a median 2 days from hospitalisation. This study was
undertaken to determine the predictive parameters for a worsening clinical course in patients with mild COVID–19
pneumonia admitted at a relatively early phase from symptom or disease onset.
Early case series from the Korean Cohort Study showed that 21.4% of patients received oxygen therapy, with no
requirement for MV [14]. In the Daegu and Gyeongbuk area, Republic of Korea, substantial community outbreaks were
linked to a large religious meeting, and a retrospective study from that area (mean age of 55.4±17.1 years) showed
that 13.3% of patients required ICU admission with MV use in 11.2%, ECMO in 4.1%, and a mortality rate of 5.1% [15].
The fatality rate of South Korea, which is 2.32% (to date) nationwide, is much lower than the rates reported in
China or other countries [4, 6, 16–19]. Although recent data (median age of 51 years) outside Wuhan, China showed an
0.8% mortality rate [20]. Studies on COVID–19 have been limited to patients with severe or critical disease admitted
to the ICU. In contrast, a recent report showed that more than 80% of COVID–19 cases were mild in China [4, 6, 13,
16–19]. In our study, most cases were mild, as evidenced by relatively low rates of ICU admission and mortality
rates that would be related to younger ages (mean 44.8 ± 17.4) and lower comorbidity rates (26.5%) than that of
previous reports. In metropolitan Seoul, all confirmed cases were either hospitalised at dedicated hospitals or
monitored at community isolation centres. Many cases of COVID–19 were diagnosed by surveillance testing in South
Korea, so that a large proportion of the patients in our cohort had mild or subclinical symptoms at the beginning of
hospitalisation. The comprehensive hospital management with close monitoring led to mostly favourable outcomes, in
addition to early case detection and immediate admission strategy of South Korea.
To our knowledge, markers for the prediction of worsening oxygenation among patients with initially mild COVID–19
pneumonia are not defined. In this cohort, only 16.9% showed fever based on a BT ≥37.5°C. Most of the first recorded
vital signs were within normal ranges and showed no difference between the two groups, suggesting that the worsening
clinical course cannot be distinguished by initial vital signs. Apart from that, we noted that more patients in the
desaturation group had higher pneumonia severity scores, and several laboratory parameters such as lymphopenia,
higher ferritin and LDH, as well as older age and comorbidities such as dementia and obesity were associated with a
worsening clinical course and oxygen desaturation. Feng et al. suggested that the variance of the MuLBSTA score, may
have a better predictive value in COVID–19 pneumonia as with this study [10, 21]. Xie et al. suggested that
traditional method such as the new early warning score (NEWS) may not help predict patients who will develop
respiratory failure in COVID–19 [22]. Our data showed no difference in MEWS.
Our results also underline the importance of the integration of CT extent and severity into the management of
COVID–19 pneumonia patients. Despite recent advances in molecular viral characterisation, recent evidence does not
support a severity or mortality stratification based on viral load dynamics [23]. However, CT extent or CT features
were recognised as important factors for predicting a worsening prognosis [23–25]. This is consistent with the
results of the current study. The rapid development of AI has significantly improved automatic lung segmentation
technology, making it possible to quantify the lesions automatically [25]. Quantitative analysis of CT using AI
tools could provide an automatic and objective estimation of the disease burden, facilitating imaging interpretation
during the pandemic [23]. This study aimed to investigate the capability of quantitative CT imaging features
combined with traditional clinical biomarkers in predicting progression to worsening course in the early stages of
COVID–19. We used free software and did not analyse the parameters for each lobe. We found that the AI-driven
parameters of pneumonia volume and extent of the whole lung in the initial chest CT predicted the worsening oxygen
saturation in mild COVID–19 pneumonia. Our study confirms the additional benefit of the visual severity scores
summing the estimated extent of each lobe and the integration of clinical and laboratory parameters.
We experienced that hypoxemia was the first step for quick deterioration in some patients and observed that some
patients are relatively asymptomatic although they have a good degree of hypoxemia for inexplicable reasons,
referred to as “silent hypoxemia” in Wuhan [22]. We designed this study to identify parameters at an early stage to
predict whether oxygen will decrease. This study has limitations. First, it is a single-centre retrospective study.
In our hospital, the standard cutoff value for admission or transfer to the ICU and change or add-on of antiviral
agents was 94% of the SpO2 on ambient air. These criteria should be regarded as empirical, as there is no
robust supporting evidence. Second, fewer patients were severe or critical compared to other studies. The majority
of the desaturation group recovered without oxygen or MV, or ECMO. These favourable outcomes may not reflect the
full prognosis of patients with COVID–19 in other areas. We did not analyse the factors that could affect outcomes
such as regional differences, medical resources, and treatment options we applied as in a clinical trial.
Nevertheless, these data, including patients with mild symptoms in a relatively early stage, are thought to be
useful in predicting parameters for clinical course and triaging patients in real practice. Third, we did not
analyse the CT scans at the time of deterioration or follow-up, and the features or patterns with GGO or
consolidation, though the extent and burden of the total GGO/consolidation of the initial CT proved to be one of the
predictive parameters for the clinical course of mild COVID–19 pneumonia.
Our study presents initial CT parameters measured by AI or visual extent scoring as well as serum markers of
inflammation such as ferritin and LDH at admission as the best parameters for predicting worsening oxygenation in
mild COVID–19 pneumonia patients.