Another compounding issue is that they had to package the vector into two parts, which then have to both infect the same cell to get any effect. Which means you have to at least double the dose to get similar coverage compared to a single AAV vector (and actually more than double). Seems like a easy recipe for liver toxicity. Which is why most companies doing AAV therapy either target the liver or the eye (where AAV doesn't escape to the liver).

I feel like the parents were not well enough informed of the risks, and the PI rushed the therapy to be famous. Not the first time this has happened, and not the last, sadly.