Moderna's mRNA vaccine is based on one they tested for SARS-Cov-1 in 2003, and they have done long-term follow-up on those participants. They all turned into alligators after 15 years.
That's like saying: the crotch rocket is based on internal combustion engines, that have been proven safe in multiple automobiles. So crotch rockets are safe because they use internal combustion engines.
MRNA is the platform. You can do good and bad things with the same platform.
More like saying, the Ninja 200xcr is a minor update of the Ninja 199xcr, which has been around since 2003 and shown to be safe when properly operated. So, Ninja 200xrc is almost certainly safe too.