Better stop the kids from eating shellfish and the larger fish - and probably a lot of other food - due to the worry of mercury poisoning then.

Food Standards Aus & NZ recommend...
"Limit to one serve (150g) per fortnight – billfish (swordfish, broadbill and marlin) and shark (flake), with no other fish eaten in that fortnight.
Limit to one serve (150g) per week – orange roughy (deep sea perch) or catfish, with no other fish eaten that week.
Eat 2–3 serves per week – of any other fish or seafood (for example, salmon or tuna).
Note: 150g is equivalent to approximately two frozen crumbed fish portions."
The implication IMO is that small amounts of mercury are in most fish and if not eaten in quantities above the recommendation then we are fine.....
Take whatever you want out of that but maybe the scare of mercury in the immunisation is not a big problem.