Flu and Covid-19: what are the risks of co-infection?

Health restriction measures aimed at reducing the spread of SARS-CoV-2 were also effective against the circulation of other respiratory viruses such as influenza. At the time of the lifting of these measures in most countries, patients are likely to be infected with several respiratory viruses at the same time. This has already been observed in some patients with co-infections with flu virus and SARS-CoV-2. What is the prognosis for these patients? A team looked into the subject and published their results in the journal The Lancet on March 25, 2022.

A study on SARS-CoV-2 and respiratory virus co-infections

In this study, 212,466 English patients were included. All of them were admitted to a hospital between February 6, 2020 and December 8, 2021 for SARS-CoV-2 infection. Some 6,965 patients received diagnostic tests for other respiratory viruses. Among them, 583 patients tested positive:

• influenza virus (n=227);
• respiratory syncytial virus (n=220);
• Has adenovirus (n=136).

More risk of serious forms and death

The authors highlighted that patients infected with SARS-CoV-2 and the influenza virus had a higher risk than a mechanical ventilation invasive procedure is implemented during their hospital stay compared to patients infected only with the Covid-19 virus. This was not the case for co-infections with SARS-CoV-2 and an adenovirus or respiratory syncytial virus. In addition, patients co-infected with the influenza virus and Covid or co-infected with an adenovirus and SARS-CoV-2 had a higher risk of death. This was not the case for co-infections with SARS-CoV-2 and respiratory syncytial virus.

Some limitations

The authors point out the limitations of their study. Patients with the most difficulty breathing are more likely to benefit from diagnostic testing for other respiratory viruses. However, after statistical correction of this bias, the excess risk of installation of invasive mechanical ventilation during co-infection by SARS-CoV-2 and the influenza virus remains.

In addition, patient vaccination data, whether against the influenza virus or against SARS-CoV-2, is not available. It would have been interesting to see whether vaccinated people had less severe clinical forms of co-infection than unvaccinated people. Nevertheless, the authors conclude that their work represents an additional argument in favor of the vaccination of populationsagainst SARS-CoV-2 and against the influenza virus as the next one approaches winter. Additionally, testing for both SARS-CoV-2 and influenza virus could be helpful in identifying patients most at risk of severe illness.