As someone working at a vaccine company, I am the opposite of a science denier. We seem to agree on most things here.

The original point was narrowly scoped, specifically as a criticism of Moderna or perhaps all vaccines approved under emergency authorization. I am not aware of any specific risk, and so I accept them as safe, but at the same time I haven't gone out of my way to do any kind of deep research on it. So personally I'm ambivalent about the criticism of moderna - but I don't wish the stifle it or paint the person as a crazy anti-vaxer.

If we are talking about MRNA vaccines we are talking about over 1.9 billion doses world wide. When these would have had any significant side effects we would have noticed these on a very, very big scale by now.

So if concern about immidiate side effects is irrational how about delayed effects? So the idea that "something" about MRNA vaccines would produce unforseen side effects long (years?) after the agent has left the body. I am not an expert by any strech, but what I read about how RNA (even our own natural one) is dealt with in our body it is extremely unlikely there would be any mechanism to execute this kind of delayed effect. Natural virus RNA enters our bodies since we exist as human beings – if foreign RNA would have dramatic delayed effects we would observe this constantly. If the spike protein RNA from the MRNA vaccine is supposed to give you delayed side effects on a grand scale, why would the same RNA from the actual virus not do it?

Are delayed side effects suddenly popping up something we observed in the history of medicine? So e.g. people getting a fever with 18 and then ten years later suddenly dying. If this would happen regularily, we would certainly have figured out that a big percentage of those who suddenly died had the fever in common etc. But such delayed effects are not something I ever heard of – if you have a long term effect of an infection (or vaccination) the "long term" doesn't mean the damage shows up after a long term. What it means is that you have the damage right away, but it will stay with you for a long term.

Of course the damage of such long term effects might be such, that it doesn't show any negative effects right away, only years later when your body gets weaker or some other conditions change. But we vaccinated over 1.9 billion people, the law of big numbers would mandate that there would have been many people where these hyptothetical long term effects would have shown negative consequences right away (e.g. if you are old and weak etc), but such negative consequences did not show up at a significant number (quite the opposite actually, traditional vaccines had and still have more of those).

The only thing left after that are vague feelings that you don't trust science or that you can never know what happens in the future etc. But if one makes that argument, it has to count for the actual virus as well, right? We don't know what happens to those infected by the virus in the long term as well. But for the actual virus there is at least evidence of negative long term effects (long covid).