Respiratory Viral Coinfections: Impacts on Virus Replication and Host Immune Response

Viral coinfections can affect viral pathogenicity, immune responses, and disease symptoms. Coinfection may occur among viruses that target the same cells, and it is more common in respiratory viruses due to their co-circulation within the population. Respiratory virus co-circulation is a major driver of coinfection and refers to the simultaneous presence and transmission of multiple respiratory viruses among humans. Coinfection with respiratory viruses, such as influenza and SARS-CoV-2, may cause more severe symptoms, complicate treatment, and increase the burden on healthcare systems. Viral coinfection often modifies immune responses to a secondary infection. Therefore, understanding the effects of respiratory viral coinfections may play an important role in reducing mortality and improving strategies for controlling viral infections. In this review, we integrate experimental and clinical findings to clarify how the sequence of infection and innate immune activation determines the outcomes of respiratory viral coinfections.