The patients with SCID, including JAK3 deficiency, have a noticeably high risk for severe, disseminated, and even fatal infections after inoculation of live-attenuated vaccines (5, 8–11), particularly Bacille Calmette–Guérin (BCG), being an active attenuated Mycobacterium bovis vaccine (5, 8, 10, 11). Here, JAK3 is linked to infection.