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Covid-19 mixed with flu raises risk of severe illness

Co-infection with flu in patients with Covid-19 more than doubles their risk of death, hospital data shows.

Adults in hospital who have Covid-19 and flu are at much greater risk of severe disease and death than those who have Covid-19 alone or with other viruses, research shows.

Patients with co-infection of SARS-CoV-2, which causes Covid-19, and flu viruses were more than four times more likely to need ventilation support and almost two and half times more likely to die than if they had Covid-19 only, a study found.

The findings show the need for greater flu testing of Covid-19 patients in hospital and highlight the importance of full vaccination against both Covid-19 and the flu, according to researchers.

Large-scale study

The team from the Roslin Institute, University of Edinburgh’s Centre for Inflammation Research and Usher Institute, University of Liverpool, Leiden University and Imperial College London, made the findings in a study of more than 305,000 hospitalised patients with Covid-19.

The research – delivered as part of the International Severe Acute Respiratory and emerging Infection Consortium’s (ISARIC) Coronavirus Clinical Characterisation Consortium – is the largest study to date of people with Covid-19 and other endemic respiratory viruses.

ISARIC’s study was set up in 2013 in readiness for a pandemic such as this. The team looked at the data of adults who had been hospitalised with Covid-19 in the UK between 6 February 2020 and 8 December 2021.

Severe outcomes

Test results for respiratory viral co-infections were recorded for almost 7,000 patients with Covid-19. Some 227 of these also had flu, and they experienced significantly more severe outcomes.

In the last two years we have frequently witnessed patients with Covid-19 become severely ill, at times leading to an ICU admission and the employment of an artificial ventilator to help with breathing. That an influenza infection could give rise to a similar situation was already known, but less was understood about the outcomes of a double infection of SARS-CoV-2 and other respiratory viruses.

Dr Maaike SwetsPhD student, the Roslin Institute and Leiden University

We found that the combination of Covid-19 and flu viruses is particularly dangerous. This will be important as many countries decrease the use of social distancing and containment measures. We expect that Covid-19 will circulate with flu, increasing the chance of co-infections. That is why we should change our testing strategy for Covid-19 patients in hospital and test for flu much more widely.

Professor Kenneth BaillieProfessor of Experimental Medicine, University of Edinburgh

The findings have been published a research letter in The Lancet.

The research was funded by UK Research and Innovation (UKRI) and by the Department of Health and Social Care through the National Institute for Health Research (NIHR) as part of the UK Government’s Covid-19 rapid research response. 

** The Roslin Institute receives strategic investment funding from the Biotechnology and Biological Sciences Research Council and it is part of the University of Edinburgh’s Royal (Dick) School of Veterinary Studies. **

Related links

Scientific publication

Roslin Covid-19 research