The study, which looked at more than 200 patients infected with both SARS-CoV-2—the virus that causes COVID-19—and the influenza virus, found patients with co-infection were four times more likely to require ventilation and 2.4 times more likely to die than if they only had COVID-19.

The study’s researchers advised COVID-19 patients to be routinely tested for the flu, as both the influenza virus and SARS-CoV-2 damage epithelial cells and cause inflammation, which together can have fatal consequences.

Professor Kenneth Baillie of Edinburgh University said in the Guardian, “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 study was led by researchers at Edinburgh University, Liverpool University, Imperial College London and Leiden University in the Netherlands, and looked at more than 212,000 patients who were hospitalized with COVID-19 between February 2020 and December 2021.

It found a total of 583 patients who had co-infection of COVID-19 with another respiratory virus. Of the 583 patients, the 227 who specifically had the influenza virus suffered from significantly more severe outcomes, the study found.

The study had a few limitations, including the risk of selection bias, as it was only conducted on patients unwell enough to be tested for possible co-infection with the influenza virus. But its findings still warrant action say the researchers, who advise people to get fully vaccinated and boosted against COVID-19 and be jabbed with the annual flu shot.

Professor Peter Openshaw of Imperial College London said, “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 hospitalized with a respiratory infection.”

The flu and COVID-19 have a lot in common: they both cause fever, cough and body aches; they both result in pneumonia: and both can be asymptomatic, mild, severe or fatal.

But where they differ is death toll. The World Health Organization estimates that between 290,000 to 650,000 people die of flu-related deaths each year worldwide. The COVID-19 virus, which continues to evolve, is thought to be 10 times deadlier than most strains of the flu, according to researchers at Johns Hopkins Medicine.

The study comes at a time when cases in COVID-19 are again rising across Europe. As the Omicron BA.2 variant—the more transmissible form of the Omicron strain—drives a resurgence of cases in the U.K., Germany, Austria, the Netherlands, Switzerland and France, many countries have also been loosening restrictions, leading to warnings from the WHO that they were lifting restrictions too “brutally.”