IFNL1 and viral infectious disease: Interestingly, IFNλ1 only exerted antiviral activity in vitro against SARS-CoV-2 at doses of 100–500 ng/mL when given before the viral infection and not as a therapeutic post-infection, while IFNλ1 given pre- or post-infection at 100 ng/mL showed restriction of influenza replication (50).