Virus-virus Interactions Among Patients Admitted to the Hospices Civils de Lyon, During 2022 to 2025 Winters Seasons

Respiratory viruses circulate throughout the year, and particularly during the winter. Consequently, co-infections involving two or more viruses can occur. The study of virus-virus interactions at the population level is necessary to understand the dynamics of co-infections. Viruses such as IAV (influenza A virus) and rhinoviruses (common cold viruses) are known to have a negative interaction, mutually excluding one another, which results in a reduced likelihood of co-infection. The study of virus-virus interactions at the population level constitutes an essential first step towards understanding and characterising these phenomena at the individual level, and subsequently at the cellular level. Using integrative approaches combining epidemiological data, clinical observations and in vitro models, it has been shown that IAV and SARS-CoV-2 have a negative interaction, whilst potentially contributing to the severity of respiratory infections. This research also suggests that prior vaccination against IAV reduces the risk of severe forms of the disease. Furthermore, viral co-infections appear to influence the severity of respiratory infections, particularly in children. Children appear to be more susceptible to these co-infections, with around 35% of cases reported, compared with 6% in adults. In this context, the study and characterisation of virus-virus interactions appear essential to better understand the pathophysiology of respiratory infections and improve the management of co-infected patients. Every week, Dr Jean-Sébastien Casalegno publishes the Hospices Civils' epidemiological bulletin, which considers the prevalence of respiratory viruses detected at the Centre de Biologie et Pathologie Nord. This local dataset reports on the co-circulation of respiratory viruses and is of interest for the study of virus-virus interactions at a population level. The impact of viral co-infections on the severity of respiratory infections remains a subject of debate in the l