Suffering with Covid and the flu at the same time increases risk of severe disease or death, according to new research.
The study found that adults in hospital with coronavirus and influenza are more likely to become more ill compared to patients who have Covid alone or with other viruses.
The researchers found that those with SARS-CoV-2 - which causes Covid-19 - and influenza viruses were four times more likely to require ventilation support and 2.4 times more likely to lose their life.
The study - carried out by researchers from the University of Edinburgh, University of Liverpool, Imperial London and Leiden University in the Netherlands - analysed more than 305,000 people who required hospital treatment after contracting Covid.
The findings suggested that there is a need for greater flu testing of Covid patients in hospitals as well as highlighting the important of full vaccination against both coronavirus and the flu.
Professor Kenneth Baillie, Professor of Experimental Medicine at the University of Edinburgh, said: "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."
The team looked at the data of adults who had been hospitalised with Covid-19 in the UK between February 6 2020 and December 8 2021.
Test results for respiratory viral co-infections were recorded for 6965 patients with Covid-19.
Some 227 of these also had the influenza virus, and they experienced significantly more severe outcomes, researchers found.
Professor Calum Semple, Professor of Child Health and Outbreak Medicine at the University of Liverpool, said: "We are seeing a rise in the usual seasonal respiratory viruses as people return to normal mixing.
"So, we can expect flu to be circulating alongside Covid-19 this winter. We were surprised that the risk of death more than doubled when people were infected by both flu and Covid-19 viruses.
"It is now very important that people get fully vaccinated and boosted against both viruses, and not leave it until it is too late."
The research, delivered as part of the International Severe Acute Respiratory and Emerging Infection Consortium's (Isaric) Coronavirus Clinical Characterisation Consortium, is said to be the largest ever study 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.
Professor Peter Openshaw, Professor of Experimental Medicine at Imperial College London, said: "Being infected with more than one virus is not very common but it's important to be aware that co-infections do happen.
"The vaccines that protect against Covid-19 and flu are different, and people need both. The way that these two infections are treated is also different so it's important to test for other viruses even when you have a diagnosis in someone who is hospitalised with a respiratory infection.
"This latest discovery by the Isaric consortium again adds significantly to improving the way we manage patients."
The findings have been published in a research letter in The Lancet.
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