The following article is a part of conference coverage from the IDWeek 2021, being held virtually from September 29 to October 3, 2021. The team at Infectious Disease Advisor will be reporting on the latest news and research conducted by leading experts in the field. Check back for more from the IDWeek 2021.

An analysis of real-world data found that the prevalence of COVID-19 with influenza co-infection was decreased among both the general population and hospitalized patients; however, those hospitalized with both infections were at increased risk for severe COVID-19. These findings were presented at IDWeek, held virtually from September 29 to October 3, 2021.

It has become well-established during the COVID-19 pandemic that   SARS-CoV-2 infection can cause increased morbidity and mortality outcomes among certain patient populations. It remains unclear, however, whether co-infection with other respiratory infections, such as influenza, increases the risk for poor clinical outcomes.

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Data for this study were sourced from the Optum De-Identified COVID-19 Electronic Health Record database. This database comprised information from patients (N=549,532) in the United States with a laboratory-confirmed COVID-19 diagnosis between February 2020 and January 2021. Patients infected with SARS-CoV-2 were assessed for a laboratory-confirmed influenza diagnosis within 10 days of testing positive for COVID-19.

A total of 1794 (0.3%) patients with SARS-CoV-2 infection were found to be co-infected with influenza. Among a subset of patients (n=80,192) hospitalized with SARS-CoV-2 infection only, 0.3% (n=242) were co-infected with influenza. In an adjusted sensitivity analysis, the researchers noted decreased rate of co-infection among both groups: 0.1% among patients in the overall cohort and 0.2% among those who were hospitalized.

Of patients who were hospitalized with SARS-CoV-2 infection, co-infection with influenza was associated with significantly increased rates of respiratory failure (56.2% vs 46.6%; P <.01). A univariate analysis showed that co-infection with influenza tended to increase the likelihood for requiring invasive mechanical ventilation among patients who were hospitalized vs the overall cohort (12.8% vs 9.8%; P =.14). In addition, co-infection with influenza was associated with an increased likelihood for transfer to the intensive care unit among patients who were hospitalized vs the overall cohort (27.3% vs 23.1%; P =.13), however, the increase was not statistically significant.

Of note, influenza co-infection did not significantly increase mortality rates among the subset of patients who were hospitalized vs the overall cohort (13.3% vs 13.0%; P =.97).

This study was limited by the relatively decreased prevalence of circulating influenza infection during the study period.

The researchers concluded that a co-infection of SARS-CoV-2 and influenza was relatively rare among patients in the US. Patients who did become infected with both viruses were at increased risk for respiratory failure and tended to require invasive mechanical ventilation and intensive care. In addition, mortality rates were not significantly impacted by influenza co-infection among patients in either group.

Disclosure: Some authors declared affiliations with industry. Please see the original reference for a full list of disclosures.


Chawla D, Chen X, Kuhlbusch K, Zalocusky K, Rizzo S. Prevalence of influenza co-infection in a real-world cohort of COVID-19 patients in the U.S. Presented at: IDWeek; September 29 to October 3, 2021.Poster 281.

Visit Infectious Disease Advisor’s conference section for coverage of IDWeek 2021.