– 4′-fluorouridine (4′-FlU, EIDD-2749) is a ribonucleoside analog that inhibits respiratory syncytial virus (RSV), related RNA viruses, and SARS-CoV-2 with high selectivity index (EC50/CC50 of ?1877) in cells and human airway epithelia organoids. 4′-FlU induces a delayed stalling of RSV and SARS-CoV-2 RdRP (RNA-dependent RNA polymerase).