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With the COVID-19 crisis, the question of viral co-infections has become a major issue. What do respiratory viruses do when they meet and what are the consequences for the animal host? The analysis of the consequences of the encounter of swine influenza viruses (Influenza A virus - IAV) with the porcine respiratory dysgenesis syndrome virus (vSDRP), a virus from a viral family (Arteriviridae), which is fairly close to that of the coronaviruses, is a relevant model for understanding these coinfections. The pig is in fact not only a species of major agronomic interest but also a model for humans. The viral interference is strong between these viruses whatever the cell type considered. Remarkably, the epithelial cell, the preferred target of the influenza virus, will react very differently to the influenza virus if it has been pre-exposed to vSDRP. Indeed, it becomes almost resistant to infection by the IAV at various levels. This observation, valid for both wild and vaccine viruses, is a key step in understanding the course of polyinfections and may have important implications for vaccine strategies.

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Different types of interactions between IAV and vSDRP are possible. 1) IAV and vSDRP can bind to each other outside the cell or, 2) and 4), to the other virus when one of the two viruses has adhered to its target cell. The influenza virus can also interact with its receptor without being influenced by vSDRP

In animal husbandry, respiratory diseases are responsible for significant economic losses [1]. These disorders are frequently due to a combination of bacterial and viral agents and are then grouped together under the name of "Porcine Respiratory Complex" (PRC) [2]. Among the pathogens involved, porcine respiratory disease syndrome virus (PRDSV) and porcine influenza A viruses (IAV) are considered major players [3-7]. While data on the pathophysiology of PRRS and IAV mono-infections are abundant, co-infection with these two viruses has been little studied [1]. PRRS infection is a persistent infection lasting several weeks due to the ability of the virus to alter the antiviral immune response [6], whereas infection with influenza virus is typically an acute infection.