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.