You can't be certain until you die (hopefully many, many decades from now) that there were no side effects. But we can be "reasonably certain" based on the sum of knowledge and data we have regarding vaccines in particular and medicine in general. Waiting 3 years, from a scientific perspective, is completely arbitrary. The length of clinical trials is not arbitrary.

Is there something specific that comes to mind when you speak of this knowledge and data on vaccines that makes you so assertive or are you just paraphrasing something you may have heard elsewhere?

Yes, I'm just saying that the more time passes, and the more people see that their friends and neighbors are suffering no side effects, that this particular concern/objection will decline.

It was very naive to think that 100% of the population would enthusiastically be on board with getting this vaccine. That never happens. I am too young to remember but I would guess there were holdouts on the first polio vaccines as well (though maybe not as many, as it affected mostly children and the diesease had much worse outcomes).

100% would be naive, but you'd think 80% or more to be a realistic expectation...

Apparently, the lengths of _these_ clinical trials were arbitrary though. See https://news.ycombinator.com/item?id=27987904