I don't believe that it's impossible. Based on what we do know it
seems impossible, but my point is that we don't necessarily know enough yet to be fooling around with the basic building blocks of human life on a widespread scale.
I think that in time the use of mRNA Covid vaccine is going to show some unexpected effect somewhere, for there's nothing in this world which does not affect something else. What that may be and how much effect it has I have no idea, but we've made plenty of mistakes thinking something was safe then later discovered it wasn't.
If I were hugely worried about it I wouldn't have accepted my Moderna jab. It's more of a philosophical point in our approaches to matters like these
Phil