> All of the previous mRNA drugs failed testing due to safety concerns.

Sure, and all the planes leading up to the Wright Flyer failed.

https://www.nature.com/articles/d41586-021-02483-w has a great timeline graphic of the various advances and breakthroughs required to get to this point, including the first drug using the lipid nanoparticles - key to fixing the safety concerns! - approved in 2018.