COVID-19 causes more severe disease than seasonal influenza, study finds

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Nearly twice as many people were admitted to hospital for the novel coronavirus (COVID-19) at the height of the pandemic than were for influenza at the peak of the 2018 and 2019 flu season, according to a new study published in The Lancet Respiratory Medicine.

The study compared data from COVID-19 patients admitted to hospital over a two-month period in spring 2020 with influenza patients admitted over a three-month period during the seasonal flu outbreak of 2018 and 2019. The study was based on data from the French national administrative database that includes details for all patients admitted to either public or private hospitals in France, including information about why they were admitted and the care they received during their stay. The researchers compared hospital admissions with COVID-19 between March 1, 2020 and April 30, 2020 with seasonal flu hospital admissions between December 1, 2018 and February 28, 2019.

Researchers found that the death rate among COVID-19 patients were almost three times higher than the death rate among influenza patients, 16.9 percent versus 5.8 percent, respectively. In addition, a greater proportion of COVID-19 patients experienced a severe illness requiring intensive care than those with influenza, with 14,585 patients admitted to the intensive care unit (ICU) with COVID-19 versus 4,926 with influenza.

The authors note that the difference in hospitalization rate may be partly due to existing immunity to influenza in the population, either as a result of previous infection or vaccination. In contrast, COVID-19 is a new virus where very few people would be expected to have any previous immunity. Nevertheless, they said their findings reinforce the importance of measures to prevent the spread of both diseases and are particularly relevant as several countries prepare for the COVID-19 pandemic to overlap with outbreaks of seasonal influenza.

Overall, disease was more severe for patients with COVID-19 compared with seasonal influenza. Patients with COVID-19 were twice as likely as flu patients to require invasive mechanical ventilation during their hospital treatment, 9.7 percent versus 4 percent. Further, the average length of stay for COVID-19 patients in intensive care was nearly twice as long as for seasonal influenza patients, with mean length of ICU stay of 15 days for COVID-19, versus 8 days for influenza.

More than one in four patients with COVID-19 experienced acute respiratory failure, where the lungs are unable to get oxygen into the body, compared with less than one in five patients with influenza (27.2 percent versus 17.4 percent). In line with previous reports, the most common underlying medical conditions among patients admitted with COVID-19 were high blood pressure (33.1 percent), being overweight or obese (11.3 percent), and diabetes (19 percent).

The authors said there are several limitations to their study. Notably, testing practices for influenza are likely to have been variable across hospitals whereas testing for COVID-19 may have been more standardized. This may account for some of the increased numbers of patients admitted to hospital with COVID-19 compared with seasonal influenza. Additionally, it is not possible to say whether the 2018/2019 flu season is representative of all seasonal influenzas, although the authors note that it was the most severe season in the past five years in France.

"Taken together, our findings clearly indicate that COVID-19 is much more serious than seasonal influenza,” said Pascale Tubert-Bitter, PhD, lead author and research director at L'Institut National de la Santé et de la Recherche Médicale (Inserm) from the University Paris-Saclay, France, in a statement. “At a time when no treatment has been shown to be effective at preventing severe disease in COVID-19 patients, this study highlights the importance of all measures of physical prevention and underlines the importance of effective vaccines.”

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