AR IFNAR2 deficiency was found in 5 patients from 4 families who presented with lethal encephalitis or hemophagocytic lymphohistiocytosis (HLH) following exposure to live attenuated MMR vaccine or with viscerotropic disease after live attenuated yellow fever virus (YFV) vaccination (20, 21, 27, 28). This evidence concerns the gene AR and hemophagocytic syndrome.