Background: Till the moment of editing this paperwork, WHO announced that there is no specific drugs or vaccines for treatment or prophylaxis of COVID-19. Additionally during the global gradual unlocking of community restrictions, WHO warned form another wave of the disease during the next few months. So clinicians and radiologists during this second wave would face great challenges in differentiation between COVID-19 and other virulent influenza viruses, mainly H1N1.

Objective: to establish comparative radiological study between COVID19 and H1N1 to find differentiating criteria that would help during their expected clinico-laboratory and radiological overlap in next Influenza season.

Material and methods: Retrospective study was including 130 patients; 65 COVID-19 patients and 65 H1N1 patients. HRCT findings to be analyzed by three expert consultant radiologists.

A. Study population and medical records review:

Inclusion criteria: 65 COVID-19 and 65 H1N1 patients to be enrolled. Exclusion criteria: (1) Degraded quality of CT scans due to patient unavoidable tachypnea with respiratory motion artifacts. (2) Unremarkable CT scans were also excluded. (3) Past history of large airway disease, asthma, COPD or bronchiectasis. (4) Patients with secondary bacterial infection.

B. CT scanning and parameters:

CT examinations conducted using multiple MDCT machines including: GE LightSpeed Plus 4 slice CT scanner (USA), Philips Brilliant-16 (USA), Siemens SOMATOM Emotion 16 and Siemens SOMATOM Sensation 64 (Germany), Toshiba Aquilion 64 and Toshiba Aquilion CXL/CX 128 (USA).

CT Scanning parameters : Slice thickness: 1 - 1.25 mm. Volumetric HRCT table speed with least cycles of breath holds as possible. Tube rotation: 0.6-0.9 second. Detector Collimation 1 mm. Helical mode (volumetric HRCT). kVp and mA per slice: 120 - 130 kVp and 200-400 mA, according to the type of MSCT machine used, the weight of the patient and the clinical indication.

C. CT analysis:

CT images to be assessed by three consultant radiologists (having long time experience in chest imaging). Image analysis in axial, sagittal and coronal planes done using both maximum intensity projection (MIP) and minimum intensity projection (Min-IP) reconstructions. The following CT features to be compared between each pathological process:

A. Site of the pathology: unilateral or bilateral - focal, multi-focal or diffuse.

B. Mosaic pattern; including ground glass opacities (GGO), mosaic perfusion, air trapping and "head cheese pattern".

C. Reticular pattern; including interlobular septal thickening, "crazy paving pattern", bronchial wall thickening, mucous plugging, traction bronchiectasis/bronchiolectasis and honeycombing.

D. Nodular pattern; including GG nodules and solid nodules; including solid nodules with "halo sign" and tree in bud nodules.

E. Relevant CT findings: including pleural, pericardial and nodal lesions.

D. Statistical analysis:

The prevalence of HRCT findings estimated as the percentage of patients showing each criteria or abnormality.

Inclusion Criteria:

• Patients with COVID-19 or H1N1

Exclusion Criteria:

(1) Degraded quality of CT scans due to patient unavoidable tachypnea with respiratory motion artifacts. (2) Unremarkable CT scans were also excluded. (3) Past history of large airway disease, asthma, COPD or bronchiectasis. (4) Patients with secondary bacterial infection.

130 patients; 65 patients known with COVID19 (registered during the period from February till May 2020) and 65 patients known with H1N1 (registered during the period from January 2010 till January 2020).

The investigators will recommend further large group future researches for some notices in the study, but preferably not be biased