Is COVID-19 finally just a bad flu? Follow-up study comparing disease severity among COVID-19 and seasonal influenza hospital in-patients across pandemic waves in Ireland

Abstract

Objectives

Coronavirus disease 2019 (COVID-19) was a more severe illness than seasonal influenza in hospitalised cohorts during the early phase of the pandemic. This study's aim was to determine if COVID-19 severity, relative to seasonal influenza, evolved across subsequent disease waves.

Study design

Retrospective population-based cohort study.

Methods

COVID-19 hospital episodes and seasonal influenza hospital episodes were identified using relevant International Classification of Disease (ICD-10) codes from the Irish national hospitalisation dataset. Descriptive comparative analysis of each group was carried out using Pearson's Chi-squared tests. Length of stay (LOS), intensive care unit (ICU) admission and in-hospital mortality were measured and compared using logistic regression analysis.

Results

Compared to influenza episodes, COVID-19 episodes for all ages and all waves combined, had a longer mean LOS (15.8 days, vs 11.4 days, P < 0.001); were more likely to receive ICU care (OR 1.24 95% CI 1.15–1.33, P < 0.001) and were more likely to die in hospital (OR 2.61, 95% CI 2.36-2-88). Despite the reduction in the proportion of patients with an intensive care unit (ICU) stay and dying in hospital in Wave 5 compared to the previous waves, the risk of having an ICU admission or dying in hospital remained higher in patients with COVID-19 in Wave 5 compared to those with influenza diagnosis.

Conclusion

While the severity of COVID-19 has reduced with successive pandemic waves, it remains a more severe disease than influenza. Despite changes in strain, population immunity, vaccination and treatment, policymakers and the public must continue to approach COVID-19 as more than ‘just a bad flu’.