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Comparison of the characteristics, morbidity, and mortality of COVID-19 and seasonal influenza: a nationwide, population-based retrospective cohort study

Our take —

Using a national database of all hospitalizations in France, this study compared patients hospitalized for COVID-19 in March and April 2020 to patients hospitalized for influenza during the 2018-19 flu season. Patients hospitalized with COVID-19 had approximately three times the risk of dying in hospital relative to patients hospitalized with influenza before and after standardizing by age and stratifying by individual comorbidities. Although far fewer patients younger than 18 were hospitalized for COVID-19 than for influenza, their risk of mortality was higher. Although there may have been differences in the thresholds for hospital admission between the two diseases, these results provide more evidence that COVID-19 has a substantially more severe clinical course than influenza, particularly among patients older than 50 years.

Study design

Retrospective Cohort

Study population and setting

The authors compared clinical outcomes and patient characteristics between 1) all patients hospitalized for COVID-19 in France from March 1 to April 30, 2020 (n= 89,530) and 2) all patients hospitalized for influenza in France from December 1, 2018 to February 28, 2019 (n= 45,819). Data were extracted from a national database (PMSI) of discharge summaries from all inpatients in France. ICD-10 codes recorded in discharge summaries were used to assess comorbidities and complications. All patients with COVID-19, regardless of symptoms, were hospitalized in France up to March 14, 2020; subsequently, only patients in serious condition were hospitalized. All patients were followed up until discharge. Results were stratified by age, and an age-standardized mortality ratio was calculated.

Summary of Main Findings

On average, inpatients with COVID-19 were younger (median 68 years vs. 71 years) and more likely to be male (53% vs. 47%) than inpatients with influenza. Nearly 20% of patients with influenza were younger than 18 years, compared to only 1.4% of patients with COVID-19. Although obesity, diabetes, hypertension, and dyslipidemia were more prevalent among patients with COVID-19, patients with influenza had a higher prevalence of heart failure, chronic respiratory disease, and other conditions. After admission, patients with COVID-19 were more likely to experience acute respiratory failure, pulmonary embolism, hemorrhagic stroke, and septic shock; whereas patients with influenza were more likely to experience myocardial infarction and atrial fibrillation. Intensive care admission (16.3% vs. 10.8%) and invasive mechanical ventilation (9.7% vs. 4.0%) were more likely in patients with COVID-19 , and the median ICU stay was twice as long (10 vs. 5 days) compared to patients with influenza. Patients with COVID-19 had nearly three times the risk of in-hospital mortality relative to patients with influenza (crude relative risk: 2.9, 95% CI: 2.8 to 3.0); a finding that was similar after age standardization (standardized mortality ratio 2.8) and after stratification by each of 13 comorbidities. Relative to patients with influenza of the same age groups, the in-hospital mortality rate was higher among patients with COVID-19 younger than 18 and older than 50. The risk of ICU admission was higher among patients with COVID-19 younger than 5 years relative to influenza patients in this age group, though in-hospital mortality was similar. Results were similar when restricted to hospitalizations after March 14, 2020, when only patients in serious condition were admitted to the hospital.

Study Strengths

This study drew on a national database of all COVID-19 hospitalizations in France and all influenza hospitalizations from an earlier period. Follow-up was complete for all patients. The authors performed a sensitivity analysis to test whether their results were affected by changing standards for COVID-19 hospital admission.

Limitations

The threshold for hospital admission may have differed for COVID-19 and influenza patients, complicating inferences about prognosis conditional on hospitalization. This is particularly likely during the first part of the study period, when all patients testing positive for SARS-CoV-2 infection were hospitalized regardless of disease severity. Although a sensitivity analysis restricting to the latter part of the study period produced similar results, admission practices early in the SARS-CoV-2 pandemic when hospital systems were severely strained were likely different from influenza-related hospitalizations in the referent period. Despite the use of a national database, case ascertainment for COVID-19 and influenza likely varied across institutions and regions. Use of electronic health records for classification of comorbidities probably resulted in under-ascertainment, though it is uncertain whether this under-ascertainment might be differential by disease. No multivariable adjustments were made for potential confounding variables.

Value added

This is the most comprehensive comparison of clinical outcomes between COVID-19 and seasonal influenza published to date.

This review was posted on: 27 January 2021