One thing I was confused by: why did Vertex choose to reactivate HbF instead of attempting to correct the mutation that causes sickle cell disease in the first place?
CRISPR, in the form used by Vertex, is not capable of directly repairing an existing gene. In the case of sickle cell this means directly changing the mutated nucleotide in the HbA gene. CRISPR is very capable of cutting the genome at precise locations. These cuts lead to lossy repair pathways that introduce mutations or deletions that disable the gene at the spot that CRISPR cut. So, the best you can hope for is that the CRISPR cut leads to a loss of function. It's possible to use CRISPR to introduce new sequences into the DNA, by introducing a new DNA sequence alongside the CRISPR proteins, then hoping that DNA repair "accidentally" uses the genetic sequence you put in to repair the break in the DNA. This is even less efficient than just cutting the DNA, and it would not fix the mutated HbA, so it's not really therapeutically relevant for sickle cell.
There are more recent techniques, notably prime editing, that use a modified version of the CRISPR system that can introduce changes to single bases (nucleotides) in the genome. These have some promise of directly fixing diseases caused by single mutations, but there are hurdles in terms of efficiently delivering the prime editor to the right tissues as well as efficiency of the actual repair.
