We, thus, report the safety and efficacy of combined casirivimab and imdevimab therapy for preventing the development of pneumonia and life-threatening COVID-19 in a child with AR complete IRF9 deficiency whose cells do not activate ISGF-3 or display induction of the ISGs activated by this transcription factor in response to both type I and III IFNs. This evidence concerns the gene IRF9 and pneumonia.