Study: SARS-CoV-2 and Seasonal Influenza: Similarity and Disparity. Image Credit: Lightspring / Shutterstock

The severity of COVID-19 compared to seasonal flu

In a recent study under review in the journal Archives of Virology and currently published on the Research Square* preprint server, investigators in Israel assessed disparities and similarities between seasonal influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections.

Study: SARS-CoV-2 and seasonal influenza: similarity and disparity. Image Credit: Lightspring / Shutterstock


Coronavirus disease 2019 (COVID-19) combines radiological and clinical features with influenza virus-induced respiratory illness. Unfortunately, it is difficult to determine the difference between these two viruses simply by examining their clinical presentations.

Since the start of the SARS-CoV-2 pandemic, researchers have drawn parallels between influenza and COVID-19. The severity of COVID-19 compared to seasonal flu is still a matter of debate.

Furthermore, early distinction between SARS-CoV-2 infection and influenza viruses is essential in hospitalized patients. This is especially crucial at this time as healthcare facilities deal with seasonal flu and the ongoing COVID-19 pandemic.

Additionally, epidemiological implications, possibility of airborne transmission, inadequate baseline herd immunityand different therapeutic methods emphasize the importance of distinguishing between patients infected with SARS-CoV-2 and those infected with influenza.

About the study

In the present retrospective research, scientists determined whether there were variations in clinical presentation and disease severity between influenza and COVID-19 in hospitalized patients. The study was conducted at a 1,000-bed university-affiliated tertiary care hospital treating more than two million people in northern Israel, named Rambam Health Care Campus (RHCC).

The survey included all adults hospitalized at the RHCC with confirmed COVID-19 during the second wave of SARS-CoV-2 in Israel between June 1, 2020 and August 31, 2020. These patients were compared to people admitted for influenza-induced respiratory illness between November 1, 2019 and August 31, 2020. Outcomes and clinical characteristics of hospitalized COVID-19 and influenza patients were compared.

Data, including clinical and demographic details, laboratory measurements on admission, and National Early Warning Score 2 (NEWS2), were collected from RHCC electronic medical records. In addition, Statistical Package for the Social Sciences (SPSS) version 26 was used to analyze the data.

Results and discussions

Collectively, the study results showed that a total of 152 COVID-19 patients and 136 influenza patients were included in the study.

Runny nose, cough, dyspnea, myalgia and comorbidities were more common in influenza patients than in SARS-CoV-2 patients. Admission hypoxemia, increased liver enzymes, or smoking habit were also more common in patients with influenza. Overall, people infected with influenza showed typical flu-like symptoms.

Patients with COVID-19, on the other hand, were overweight, had lymphopenia below 1500, C-reactive protein (CRP) above 5 mg/dL, or radiographic abnormalities. There is growing evidence that chronic inflammation and excess adiposity associated with obesity increase susceptibility to viral infections and disease severity due to immune system dysregulation and elevated cytokine levels pro-inflammatory.

The most common laboratory abnormality in COVID-19 patients was lymphopenia, which was 54 times more common than in people infected with influenza. Direct viral infection of lymphocytes due to the presence of angiotensin-converting enzyme 2 (ACE2) receptors on their cell membrane and induced lymphocyte deficiency due to pro-inflammatory cytokines were two possibilities that account for the significant lymphopenia in SARS-CoV-2 infection.

COVID-19 patients had a higher percentage of abnormal chest x-rays (CXR), manifesting mainly as bilateral infiltrates. This inference was consistent with several previous studies. However, this was at odds with a previous smaller study.

Furthermore, the incidence of dehydration on hospital admission was high in patients with SARS-CoV-2. Intravascular exhaustion caused by the direct impact of the virus on renal ACE2 receptors could explain this phenomenon.

Demand for ventilatory support, length of hospital stay, and 30-day mortality were similar in both groups, despite the fact that influenza patients had severe illness with a NEWS2 of more than six at l ‘admission. In contrast to the present study, an earlier investigation from Germany found that hospitalized patients with COVID-19 had increased in-hospital mortality and poorer clinical outcomes, such as length of hospital stay and ventilation, acute kidney injury and acute respiratory distress syndrome.


The results of the study revealed that patients with influenza were more severely ill during their hospital stay than patients with SARS-CoV-2. Both cohorts, however, had equal in-hospital deaths and clinical outcomes.

Moreover, the two diseases possessed distinct properties that allowed them to be separated from each other during the period of admission until laboratory diagnosis. The diverse characteristics of COVID-19 and influenza make it easier to implement appropriate prevention strategies and infection control approaches upon arrival at the hospital.

*Important Notice

Preprints with Research Square publish preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.

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