> One such thing is sudden onset symptoms well after the thing has left your system, because there isn't a biological way for that to have an effect
The hard part about conversations like this are that you could have written the definitive book on this stuff, or you could be just another dummy on the internet like me. So I have to take a statement like that and do my best to evaluate it myself, since I have no idea how to value your expertise. If you're an expert of some sort, I apologize in advance.
We know for a fact that vaccines have long term effects. They wouldn't be useful otherwise. It would be nice if they could only have positive long term effects, but why would there be any guarantee of that?
Symptoms could start immediately but be undetectable until they progress far enough (think a tiny little tumor for just one example). Again, not claiming this is happening, just saying that it seems nutty to assume it can't happen.
When I read "we understand how vaccines work pretty well", I automatically translate it to "we understand pretty well what happens when we inject a substance into a human body", and then I think "really?". We have some useful models, sure, but "understanding pretty well" seems a vast overstatement of the situation, especially when its laypeople chatting on the internet.
The abstract concept of a vaccine is pretty simple. Anybody can regurgitate the picture book version of it. Any specific instantiation of one is remarkably complex, all the way from manufacturing, to transport, to administration, and then interacting with the legacy ball of spaghetti that is the human body and it's various systems.
The fact that we humans decided to name some substance the same thing that we named some other substances that we created in the past doesn't necessarily tell you how that substance is going to interact with the body. It can provide you with hypotheses, but hypotheses need to be tested.