As any biologist knows there are exceptions to almost all rules. It's not strictly correct there is zero risk of genomic integration, it's just very low. Howeve

As any biologist knows there are exceptions to almost all rules. It's not strictly correct there is zero risk of genomic integration, it's just very low. However there are some 4500 annotated retrocopies in the human genome, so spliced mRNA did cross back into the nucleus and was integrated in the genome at least this often in the human germline (with the help of retrotransposons). Of course the likelihood of this happening, and then causing problems in vaccinated individuals are extremely small. Maybe even almost impossible as the mRNAs for the vaccines are heavily modified.