Covid-19 patients infected with flu twice as likely to die, study shows

People who are simultaneously infected with Covid-19 and influenza face double the risk of death, according to a new study, highlighting the challenge being posed to health systems as flu re-emerges.

The research, led by scientists and medics at academic institutions across Europe as part of the disease response network Isaric, found that unvaccinated patients infected with the two viruses were 2.3 times more likely to die and 4.1 times more likely to require a ventilator, compared with people only infected with coronavirus.

Geert Groeneveld, one of the study authors and an infectious disease physician at the Leiden University Medical Center in the Netherlands, said the harm from a co-infection of Covid and flu probably stemmed from how they both “destroy the respiratory tract” in severely ill patients.

Most countries have been spared large flu surges over the past two years as public health restrictions implemented to counteract the spread of Covid-19 have also suppressed the peak of influenza seasons. However, some health experts have warned that a rare spring or summer flu surge in the northern hemisphere is possible.

Researchers said the findings highlighted the importance of ensuring high uptake for both Covid-19 and flu vaccines, and the need for enhanced testing of Covid-19 patients for flu.

The research, published on Friday evening, did not however find an elevated risk of severe illness for Covid-19 patients co-infected with RSV and adenovirus, other common respiratory viruses.

Nearly 7,000 Covid patients in the UK were tested for co-infections as part of the study between February 2020 and December 2021, of which 227 people were found to have both flu and Covid at the same time. But researchers noted they were “unable to determine” the impact of vaccination on patient outcomes because most patients were admitted before jabs were widely available and they could not access flu vaccination status on patients’ records.

Calum Semple, professor of child health and outbreak medicine at Liverpool University, who contributed to the research, said co-infections could affect thousands of people during future flu surges.

“We’re coming out of Covid restrictions, flu will come back and although we’ve vaccinated against Covid we may not have so much vaccination [against] flu,” he said. Semple stressed that not all patients would die “but a lot of them are going to have a very torrid time in hospital”.

He stressed that “it is now very important that people get fully vaccinated and boosted against both viruses, and not leave it until it is too late”.

Kenneth Baillie, professor of experimental medicine at the University of Edinburgh who co-authored the study, said the risk was “a very real one for people, as flu returns in the next winter and maybe before”.

Some southern hemisphere nations, including Brazil and South Africa, have recently been hit by flu surges at unusual times of year, possibly foreshadowing a return of the virus in the northern hemisphere earlier than winter.

“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,” added Baillie.

“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.”