In this stage, clinical symptoms exhibited by patients with COVID-19 are mild to moderate, although some patients are asymptomatic. Usually there is no imaging evidence of pneumonia in patients that are asymptomatic or those with mild symptoms, and the changes of imaging are often atypical, which may result in omissions. For example, Zhang et al (24) demonstrated that high-resolution computed tomography (HRCT) exhibited multiple instances of ground-glass opacity (GGO) and may be accompanied by consolidation in patients with early stage COVID-19. The study also revealed that certain patients did not present imaging results that were indicative of pneumonia, and others exhibited normal chest radiographs, but HRCT results revealed pneumonia. Therefore, with imaging as an important supplement to the screening of COVID-19, HRCT should be recommended as the initial imaging technique, as X-rays often result in missed diagnoses in the early stage. Pulmonary CT manifestations are usually as follows: i) GGO or consolidation changes, in which multiple lesions on the bilateral lung are common. The scope of consolidation is small and localized (25); ii) the density of the lesions is uneven, and they are distributed in a localized manner. Generally, only parts of the lung segment are affected, mostly within the extrapulmonary zone and the lower lung (25); iii) there are no manifestations of mediastinal or hilar lymphadenopathy, pleural thickening or pleural effusion (26). Typical HRCT patterns of patients with early stage COVID-19 are presented in Fig. 1A and B.

SARS is an acute infectious disease with fever as the first and primary symptom, often occurring without upper catarrhal symptoms (18). At the early stage, the time from clinical symptom presentation to chest imaging abnormalities is generally only 2-3 days. X-rays and CT scans of the lungs demonstrate small or round-shaped GGO, with some patients presenting this alongside lung consolidation. Single lesions are more common, and those involving the lung segment are rare. Most of the lesions are distributed in the lower field and lateral bands of both lungs (18).

The early stage of MERS usually manifests as an acute respiratory infection. Patients with low immune function or underlying diseases, including coronary heart disease and diabetes, may have more severe symptoms, such as dyspnea (27). However, for those without underlying disease, symptoms are mild or asymptomatic, and some patients do not exhibit imaging changes (28). The primary features of the lung that are visible in HRCT images are GGO changes and occasionally mixed consolidation or small nodules, most of which are distributed in the subpleural and basal lung regions (29,30). Some cases may demonstrate varying degrees of pleural effusion (31,32).

Early imaging features of the three diseases share several similarities. GGO is the primary symptom, and a small degree of consolidation may be observed. Lesions generally do not affect the entire lung segment and are most common in the lower lung field and lateral bands. However, certain patients with COVID-19 do not present changes in chest images in the early stage, whereas patients with SARS demonstrate pneumonia within a short period of time (2-3 days). The reasons for this difference may be the duration of the viral incubation period, the method of virus detection used or the popularity of pulmonary HRCT. The reason of pulmonary HRCT being not commonly used is attributed to the cost of the CT examination and the doctors' cognitive level of the characteristics of different coronavirus diseases. The aforementioned similarities and differences of early-stage features of COVID-19, SARS and MERS are summarized in Tables I and II.

There are several pulmonary HRCT imaging features of COVID-19: i) The confluence or expansion of GGO lesions may be demonstrated, with some being accompanied by certain reticular changes, such as the ‘crazy-paving pattern’. Sometimes lesions appear as consolidations, and signs of air bronchogram may be observed. GGO can also appear around consolidations or other lung fields (26). ii) The lesion area may increase due to multiple lesions fusing together or through diffusion into multiple lung lobes, demonstrating asymmetric distribution in the lungs. This is most commonly observed feature in the middle and lateral bands. iii) Enlargement of the mediastinum and hilar lymph nodes may occur, although this is rare. The lesions progress rapidly and clear changes in imaging morphology appears within a short period (several days) (25,26). Active treatment is required and the possibility of ARDS must be considered (26). Typical imaging patterns of progressive stage COVID-19 are presented in Fig. 1C and D.

In the progressive stage of SARS, fever and other symptoms of infection persist, with imaging demonstrating progressive deterioration within 3-7 days after onset (18). The features of pulmonary CT imaging are as follows: i) GGO increases or occurs alongside consolidation, and the lesions are large or diffuse; and ii) the lesions may spread from one lung field to multiple lung fields, with lesions of the unilateral lung progressing into the bilateral lung. Lesions are distributed in multiple lung lobes, but primarily in the lower lobe, with a mixed distribution in the inner and outer lung fields (18,33). However, central distributions are rare (18). At this stage, pulmonary lesions proliferate.

From the date of onset, MERS can progress within 2-3 weeks. Additionally, certain patients may progress rapidly from asymptomatic infection to pneumonia within 4-7 days (34-36), demonstrating pneumonia-associated clinical symptoms and typical imaging changes (37). Multifocal nodular consolidation with rapid progression in the lower lung and the lateral zone of the lung may be demonstrated in pulmonary HRCT images. This is often accompanied with a GGO halo, mixed consolidation (34) and bilateral interstitial infiltration (38).

Overall, the presentation of COVID-19, SARS and MERS is similar in the progressive stage. Each demonstrates larger lesion areas, pronounced consolidation shadows and a wide distribution of the lesions. During this stage, the disease progresses rapidly and can lead to ARDS if the condition worsens. The differences observed between infections include the presentation of pleural effusions, evidence of an air bronchogram or evidence of a GGO halo. The similarities and differences in images at the progressive stage of COVID-19, SARS and MERS are summarized in Tables I and II.

A retrospective study by Guan et al (39) summarized the clinical characteristics of 1,099 patients with COVID-19 in 552 hospitals located in China. The results revealed that 15.7% of patients developed severe pneumonia. An additional study demonstrated this value to be 25.5% (40). For COVID-19 to be classified as severe, patients must meet any of the following criteria: i) Respiratory distress (respiratory rate, ≥30 breaths'min), ii) oxygenation index ≤300 mmHg, iii) finger oxygen saturation ≤93% in a resting state and iv) chest images presenting >50% lesion progression within 24-48 h (23).

Pulmonary HRCT images suggest that as the GGO density increases, the lesions fuse and progress into multiple, large and diffuse consolidations on the bilateral lung from the periphery to the center, involving multiple lobes and presenting as “white lung”. Additionally, certain patients demonstrate a small degree of pleural effusion. This phase of treatment is difficult, and the mortality rate is 49%. Certain patients may exhibit insignificant changes in imaging, despite worsening clinical symptoms. This is most common in patients with other underlying diseases, such as cerebral vascular disease (26). Typical imaging patterns of patients with severe COVID-19 are presented in Fig. 1E and F.

The majority of patients with SARS enter the very severe stage 2-3 weeks after onset. Imaging morphology and lesion range change rapidly at this stage, with some changes in chest imaging occurring within 1-3 days (33,41). Patients may demonstrate ‘white lung’ in images, which indicates that ARDS had occurred (41). ARDS may develop in 10-15% of patients with SARS (41,42), which is a life-threatening condition. The presentation of ‘white lung’ in images may indicate poor prognosis and death, but it can also disappear after treatment in certain patients (18). In addition, SARS in the severe stage is prone to relapse. The images of certain patients may indicate that the lesion has disappeared; however, it may then reappear or become aggravated in a short period of time (18).

Most severe cases of MERS progress into severe pneumonia within 1 week. This can lead to ARDS, acute renal failure, septic shock or multiple organ failure. Patients with MERS are more prone to acute renal failure than those with SARS (29,37). The WHO reported that 12.4% of patients with MERS develop ARDS (43). The primary imaging feature of this stage is bilateral interstitial infiltration that progresses rapidly (44). Furthermore, imaging typically indicates the deterioration of lesions, including those patients previously presenting with ‘white lung’. The changes in images are rapid and require attentive monitoring (30).

The differences in imaging features between COVID-19, SARS and MERS include the progression rate of lesions, the likelihood of pleural effusion, the main clinical manifestations of consolidation or GGO and whether the consolidation or GGO is the primary manifestation. The similarities and differences in images at the severe stage of these diseases are summarized in Tables I and II.

The recovery stage of COVID-19 typically occurs 1-2 weeks after the onset of pneumonia. The imaging features include a decrease in the scope and density of lesions, a gradual disappearance in consolidation lesions and the beginning of organizing pneumonia. The lesions may completely disappear, or part of the funicular shadow may remain (25,26). Changes in imaging at the recovery stage generally lag behind the improvement of clinical symptoms (25). However, the lesions may subsequently enlarge, or new lesions may appear in certain cases (25). Typical imaging patterns of patients in recovery stage of COVID-19 are presented in Fig. 1G and H.

The majority of SARS cases proceed to the recovery stage within 2-3 weeks after onset. The range and density of the lesions observed in images may exhibit a gradual decrease, or they may disappear entirely. Pulmonary fibrosis is also a common imaging feature during recovery (18). Patients with severe cases are more prone to pulmonary fibrosis compared with those with ordinary infection, with fibrosis disappearing at a slower rate (45). The majority of patients recover within 2-3 months post-discharge (18) and 7-8% of patients demonstrate pronounced sequelae of pulmonary fibrosis (46).

The imaging features of patients with MERS during the recovery stage include the scope of lesions decreasing significantly and certain patients experiencing left pulmonary fibrosis. The rate of improvement in clinical symptoms is slightly faster than what is exhibited by images (30,34). A case study reporting imaging follow-up revealed that abnormalities in multiple nodules combined with GGO declined after treatment, but progression in fibrosis was observed (34).

The overall condition of patients with COVID-19, SARS and MERS during the recovery stage tends to be stable, and images usually indicate that the lesions have gradually disappeared. In general, changes in imaging occur later than improvements in clinical manifestations. Regarding disease progression, the recovery stage of COVID-19 is earlier than that of SARS; however, some patients with COVID-19 may exhibit recurrent conditions that require attention (47). The similarities and differences in images at the recovery stage of COVID-19, SARS and MERS are summarized in Tables I and II.

The majority of chest images obtained at the first hospital visit in patients with SARS or MERS are abnormal, which aids the successful and early diagnosis of patients (18,31). However, for patients exhibiting early stage COVID-19, the interpretation of imaging results requires careful attention. Guan et al (39) demonstrated that radiologic abnormalities were not identified in the initial presentation of 17.9 and 2.9% of non-severe and severe cases, respectively. Although the detection of viral nucleic acid is the first method used to diagnose COVID-19, and while chest imaging should not replace this method, many countries are still facing a shortage of nucleic acid reagents, particularly in poor and developing countries (48,49). During the early stages of COVID-19 outbreaks or during large-scale outbreaks, the flowchart for screening COVID-19 presented in Fig. 2, which was constructed based on our previous clinical experience in Wuhan, may be used as a reference for diagnosis in the absence of nucleic acid testing kits.

A previous study has indicated that consolidation lesions could serve as a marker of disease progression or a more severe disease state following COVID-19 infection (50). Furthermore, the consolidation of lesions can be indicative of disease progression or deterioration (51). A second study also indicated that pleural effusion was identified in 33% of patients with MERS and was associated with poor prognosis (52). An observational study on the chest radiographs of 55 patients with MERS revealed higher rates of pneumothorax and pleural effusion in deceased patients compared with those who had recovered (53).

A previous study that assessed the clinical outcome of 70 patients with MERS revealed that the imaging manifestations of the bilateral lung were a risk factor for intensive care unit admission (44). Imaging manifestations of patients with severe COVID-19 include bilateral lung involvement and interstitial change, which indicates poor prognosis (39). Patients with SARS may demonstrate multiple lung lobe lesions. If the range of lesions usually exceeds one third of the lung lobe, the patient may be at the severe stage of infection (54).

Two consecutive contrast HRCT scans of the lungs that demonstrate rapid lesion progression, mainly consisting of consolidation combined with GGO and air bronchogram, may indicate that the patient is at a high risk of COVID-19 progression from a common type to severe type (55). X-rays obtained in a previous study demonstrated progression in >50% of lesions within 48 h, which was indicative of severe SARS (54).

As of December 16, 2020, the mortality rate of patients with COVID-19 has been reported to be 2.26% worldwide (2). However, among male patients aged ≥60, an initial diagnosis of severe pneumonia and a delay in diagnosis were associated with elevated mortality rates (40). Early diagnosis is an important measure for the prevention of severe pneumonia or death, and imaging examination may therefore be helpful. The majority of patients with severe cases demonstrate imaging abnormalities at the time of onset, and consolidation generally indicates disease progression. Pleural effusion, pneumothorax, bilateral lung involvement and the rapid progression of lesions can be indicative of severe cases. If necessary, chest X-rays or pulmonary CT scans should be re-examined within 48 h (25,56). Chest imaging can be used for early diagnosis, the early identification of severe cases and for early treatment guidance. It can also reduce the risk of death (25).