Updated project metadata. Inborn errors of human IFN-γ immunity underlie mycobacterial diseases, whereas inborn errors of IFN-a/b immunity underlie viral diseases. Both types of IFNs induce the transcription factor IRF1. We describe two unrelated children with inherited complete IRF1 deficiency and early-onset, multiple, life-threatening diseases caused by weakly virulent mycobacteria. These children have no history of severe viral disease, despite exposure to many viruses, including SARS-CoV-2, which is life-threatening in individuals with impaired IFN-a/b immunity. The IRF1-dependent cellular responses to IFN-γ are, both quantitatively and qualitatively, much greater than those to IFN-a/b in vitro. Monocyte- and iPSC-derived macrophages from the two patients show no upregulation of at least 20% of the target genes normally induced by IFN-γ. By contrast, cell-intrinsic IFN-a/b immunity to diverse viruses, including SARS-CoV-2, is intact. Human IRF1 is, thus, largely redundant for antiviral IFN-a/b immunity. By contrast, human IRF1 is essential for IFN-γ immunity to mycobacteria in myeloid cells.