Interactions between SARS-CoV-2 and influenza, and the impact of coinfection on disease severity: a test-negative design

Int J Epidemiol. 2021 Aug 30;50(4):1124-1133. doi: 10.1093/ije/dyab081.


Background: The impact of SARS-CoV-2 alongside influenza is a major concern in the northern hemisphere as winter approaches.

Methods: Test data for influenza and SARS-CoV-2 from national surveillance systems between 20 January 2020 and 25 April 2020 were used to estimate influenza infection on the risk of SARS-CoV-2 infection. A test-negative design was used to assess the odds of SARS-CoV-2 in those who tested positive for influenza compared with those who tested negative. The severity of SARS-CoV-2 was also assessed using univariable and multivariable analyses.

Results: The risk of testing positive for SARS-CoV-2 was 58% lower among influenza-positive cases and patients with a coinfection had a risk of death of 5.92 (95% confidence interval: 3.21-10.91) times greater than among those with neither influenza nor SARS-CoV-2. The odds of ventilator use or death and intensive care unit admission or death were greatest among coinfected patients.

Conclusions: Coinfection of these viruses could have a significant impact on morbidity, mortality and health-service demand.

Keywords: SARS-CoV-2; coinfection; epidemiology; influenza; interaction.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • COVID-19*
  • Coinfection* / epidemiology
  • Humans
  • Influenza, Human* / epidemiology
  • SARS-CoV-2
  • Severity of Illness Index