I don't get angry like some people about "anti-vaxxers", but the reason I am not one, is because I think that if low levels of mercury were a major health issue, then it would be apparent in large populations that consume a relatively large amount of mercury-containing seafood. e.g. Japan. And in Japan, there have also, as it happens, been incidents of acute mercury poisoning with significant numbers of people, so the difference between toxic and non-toxic levels is clear.