I get what you're saying, but the logical conclusion of all this is that no one can know anything at all unless they're an expert in that specific thing. But that's why we have experts in the first place - so that we can determine what is most likely to be true without necessarily needing to deeply understand it first hand. So sure, perhaps there's some small chance that the vaccines don't work the way they're supposed to or the earth is flat. But "expert consensus is X" is a pretty good heuristic for "X is correct." And while I'm not defending random trolls on Facebook, this does happen to be a pretty sensitive subject right now because there is a pretty significant proportion of the population who won't get the vaccine precisely because they don't believe in expertise.