N4-Hydroxycytidine is a ribonucleoside analog with broad-spectrum antiviral activity. The prodrug of N4-hydroxycytidine is EIDD-2801. N4-Hydroxycytidine is active against influenza, Ebola, Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV)and Venezuelan equine encephalitis virus (VEEV). It inhibits replication of SARS-CoV-2 and Middle East respiratory syndrome coronavirus (MERS-CoV) in Calu-3 2B4 cells with IC₅₀ values of 0.08 µM and 0.15 µM respectively. N4-Hydroxycytidine (100 and 400 mg/kg twice per day) reduces lung viral titers in a mouse model of Respiratory Syncytial Virus (RSV) infection.