Roughly four months after the genetic modifications, six out of seven babies are out of protective isolation and leading healthy lives, according to doctors at St Jude Children's Research Hospital. The remaining infant's immune system is still in the process of constructing itself.

The patients in the study were all born with the inherited X-linked SCID, which is limited to boys as it's triggered by a genetic defect in the male X chromosome. The treatment they received uses an inactivated form of HIV to apply genetic modifications to bone marrow -- which is prepped using low doses of chemotherapy -- in order to kickstart it to produce all three major immune cell types. "The initial results also suggest our approach is fundamentally safer than previous attempts," said lead study author Dr. Ewelina Mamcarz.

At first glance, the treatment is being viewed as a possible cure. But, more work is needed -- specifically, the babies need to be monitored to ensure they remain stable with no side effects. Their response to vaccination will also need to be tracked.