IFNG and severe combined immunodeficiency: Systemic complications usually occur within 6 months of vaccination and are most commonly seen in children with underlying primary immunodeficiency (severe combined immunodeficiency, chronic granulomatous disease, disorders of the gamma-IFN/IL-12 pathway, or a group of conditions termed Mendelian Susceptibility to Mycobacterial Disease (MSMD)) [4,17,19,28–30].