IFNαR1-deficiency (IFNAR1, OMIM #107450), due to biallelic damaging variants in IFNAR1, is an inborn error of immunity underlying susceptibility to severe disease due to live attenuated viruses in measles-mumps-rubella/varicella (MMR/V) or yellow fever vaccines [1–3], and natural infection with viruses including influenza, herpes simplex and SARS-CoV-2 [3–8]. The gene discussed is IFNAR1; the disease is measles.