You can be healthy now and developer cancer next year. They're looking at whether this child shows signs of illness, and he doesn't. That's the same definition of healthy that we use for everyone else.
You cant but you can place a very high probability on it.
With genetic diseases often the difference between healthy and not is a single enzyme is missing.
If you can prove the enzyme level is restored and all the downstream effects match healthy individuals, youve basically restored them to what healthy people are.
Durability of the change is the big issue. Some gene therapies wane over time.
And of course if damage had occurred before the gene therapy, that may never go away.
The headline claims the child is healthy, but the article itself says
> Roughly six months out from treatment, "Teddi is a happy and healthy toddler showing no signs of the devastating disease she was born with," the NHS statement reads.
and later
> In clinical trials, Libmeldy offered clear benefits to infantile and juvenile patients who hadn't yet developed MLD symptoms; these patients were able to break down sulfatides at normal rates and showed typical patterns of motor development, for example. The benefit of the therapy seemed to last several years, but at this point, "it is not yet clear whether it will persist life-long, and extended follow-up is needed," the EMA noted.
My assertion is that the every change we do towards things like that might involve million results and since it's not a simple logical statement it's rather hard to verify/validate the outcome in a short term due to the nature of dynamic/complex systems.