The amount in some flu vaccines is roughly equivalent to the amount of mercury found in a three-ounce can of tuna fish.
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overall, autism rates did not decline after thimerosal was removed from the vaccines, as many anti-vaccine groups suggested it would. Instead, the rates increased.
It’s worth noting that most flu vaccines (the majority of all vaccines, really) in the US are preservative free anyways. The CDC’s decision is still based on junk science, but it doesn’t change much in the big picture.