Comparison of COVID-19 with influenza A in the ICU: a territory-wide, retrospective, propensity matched cohort on mortality and length of stay

Abstract

Objectives: Direct comparisons between COVID-19 and influenza A in the critical care setting are limited. The objective of this study was to compare their outcomes and identify risk factors for hospital mortality.

Design and setting: This was a territory-wide, retrospective study on all adult (≥18 years old) patients admitted to public hospital intensive care units in Hong Kong. We compared COVID-19 patients admitted between 27 January 2020 and 26 January 2021 with a propensity-matched historical cohort of influenza A patients admitted between 27 January 2015 and 26 January 2020. We reported outcomes of hospital mortality and time to death or discharge. Multivariate analysis using Poisson regression and relative risk (RR) was used to identify risk factors for hospital mortality.

Results: After propensity matching, 373 COVID-19 and 373 influenza A patients were evenly matched for baseline characteristics. COVID-19 patients had higher unadjusted hospital mortality than influenza A patients (17.5% vs 7.5%, p<0.001). The Acute Physiology and Chronic Health Evaluation IV (APACHE IV) adjusted standardised mortality ratio was also higher for COVID-19 than influenza A patients ((0.79 (95% CI 0.61 to 1.00) vs 0.42 (95% CI 0.28 to 0.60)), p<0.001). Adjusting for age, P_aO₂/F_iO₂, Charlson Comorbidity Index and APACHE IV, COVID-19 (adjusted RR 2.26 (95% CI 1.52 to 3.36)) and early bacterial-viral coinfection (adjusted RR 1.66 (95% CI 1.17 to 2.37)) were directly associated with hospital mortality.

Conclusions: Critically ill patients with COVID-19 had substantially higher hospital mortality when compared with propensity-matched patients with influenza A.

Keywords: Adult intensive & critical care; COVID-19; Respiratory infections.

Figure 1

Kaplan-Meier plot of hospital death and discharge in COVID-19 and influenza A intensive care unit patients. Time to hospital death or discharge from hospital admission for critically ill patients with COVID-19 (n=373) and influenza A (n=373).

Figure 2

Kaplan-Meier plot of hospital death and discharge based on 65 years old age threshold. (A) Time to hospital death or discharge from hospital admission for critically ill patients aged ≤65 years old with COVID-19 (n=169) and influenza A (n=225). (B) Time to hospital death or discharge from hospital admission for critically ill patients aged >65 years old with COVID-19 (n=204) and influenza A (n=148).