Earlier quoted context omitted.
no significant side-effects _for now_.
The mechanism of action is known, so potential side effects are largely predictable. I'm not an expert, but I don't see any reason to think there would be unexpected long term side effects from this sort of vaccination. Is there reason to think otherwise?
So there are many reasons to think otherwise, but the simplest one is - Every single medication and vaccine that we've ever made has known side-effects. Not all side-effects are known right off the bat, and many take years to detect.