This study is looking to help babies with a very rare condition called SCID, which affects their immune system. SCID stands for Severe Combined Immunodeficiency, meaning their body can't fight off germs well. The researchers want to see if using smaller doses of chemotherapy, a strong medicine often used for cancer, can help these babies get better without as many risks. Babies will get a special treatment called a "stem cell transplant" from a donor who is not related or only partially matched with them.

Stem cells are cells that can turn into other types of cells in the body, and this might help the baby's immune system work better. The study will check how well the babies respond to vaccines after the transplant. After the transplant, the researchers will watch the babies for 3 years to see how they are doing and check if their immune systems are improving.

• Study duration: 3 years of follow-up after transplant.
• Participants: Babies with SCID using unrelated or half-matched donors.
• Goal: Improve immune recovery with lower chemotherapy doses.