COVID-19
A new form of respiratory infection was detected in Wuhan city, China, in December 2019. In less than one month, the pathogen was recognized as a novel type of Coronavirus. It was named as “severe acute respiratory syndrome coronavirus 2” (SARS-CoV-2), and the resultant infection was called COVID-19. Despite robust international measurements and quarantines in many countries, this form of infection became a pandemic in less than three months. By July 24, 2020, about 9.3 million infections and 478000 death have been reported in 188 countries because of COVID-19 (1-4). The SARS-CoV-2 is an enveloped single-stranded RNA virus that attaches to the angiotensin-converting enzyme 2 of airways with a duration of infectiousness of 3 days before the onset of symptoms until clearance of the virus. The incubation time is 2–14 days. No vaccine or standard treatment is available for COVID-19 at this time. It is unknown if the COVID-19 prevalence is different in different seasons (5).
Influenza
Influenza infection is one of the common forms of respiratory viral infection. It has been estimated that each year between 291,000 to 645,000 death happens because of the influenza infection in the world (6, 7). There are four forms of influenza virus, including A, B, C, and D. Usually, the seasonal Influenza is caused by type A and B (8). H1N1 Influenza is one of the worst subtypes of influenza infection and caused two pandemics in 1918 and 2009 with 50 million and over 280000 death, respectively (9, 10). The influenza virus is an enveloped single-stranded RNA virus that attaches to the N-acetyl neuraminic acid in the airways. Its distribution is via droplets. Duration of infectiousness is from 1 day before illness to as long as severe symptoms persist with an incubation time of 1-4 days. There are several approved vaccines and treatments for the Influenza virus, and its prevalence is seasonal (5).
Clinical differentiation between COVID-19 and Influenza is difficult. There is a substantial overlap between the clinical manifestations of Influenza and COVID-19. Fever, cough, expectoration, and dyspnea are the main manifestation of these two infections. Other clinical presentations of these viral infections are headache, sore throat, chest pain, fatigue, myalgia, nausea, vomiting, and diarrhea. Cough and expectoration have been reported more commonly in Influenza. The prevalence of other clinical findings is similar in these two diseases (6).
Also, there is a substantial overlap in laboratory findings in these two diseases. Lymphopenia elevated C-reactive protein, and erythrocyte sedimentation rate levels have been reported in both diseases without significant differences (6). The Procalcitonin level is also not significantly different between these two infections (6).
Reverse transcription-polymerase chain reaction (RT-PCR)
While the rapid RT-PCR assay is a very robust technique for diagnosis of Influenza A and B with a sensitivity of 98% and specificity of 99% (11), the same thing is not true for COVID-19 PCR. The accuracy of PCR in COVID-19 depends on the time and technique of sampling. Its false negative result is very high in the incubation phase (100% false-negative five days before initial symptoms, 67% false-negative one day before initial symptoms). PCR is falsely negative in 38% of patients on the day of initial symptoms. Its false negative rate is 20% three days after initial symptoms and 21% four days after initial symptoms (12).
CT scan
Currently, there are a few studies regarding the role of medical imaging for the differentiation of these two infections. The disease burden of COVID-19 on chest CT is reported to be higher than the Influenza A (CT score of 13 versus 6) (6). Also, it has been reported that the frequencies of bronchiectasis, pleural effusions, linear opacities, crazy-paving opacities, and vascular enlargement within the pulmonary lesions are different in COVID-19 and Influenza A pneumonia and can be potentially be used for differentiation (6). In another study, the COVID-19 patients had more rounded opacities and interlobular septal thickening on chest CT but less pulmonary nodules, tree-in-bud opacities and pleural effusion in comparison to patients with Influenza A and B (13). Even though the most CT manifestations of viral infection are nonspecific, the preliminary reports are promising about the role of CT to differentiate these two infections.
Fall and winter of 2020
It is unlikely that the vaccine and standard treatment for the COVID-19 are available by fall and winter of 2020. In this period, seasonal flu can be superimposed on the COVID-19 pandemic. Differentiation between these two infections is critical for patient management. Given the facts mentioned above and substantial overlap between clinical and laboratory presentations of COVID-19 and Influenza, CT would be critical for this task. Differentiation of COVID-19 versus Influenza on chest CT is challenging for medical centers without experienced chest radiologists. In this study, we tested the feasibility of automated diagnostic techniques based on transfer learning AI on chest CT images. Having such platforms can help physicians without chest imaging experience during the fall and winter of 2020.