Myth... The mercury received in a vaccine is no greater than in a can of tuna. Eating a can of tuna has certainly never caused autism...
This myth has received a lot of publicity because it offers an analogy anyone can understand and makes the mercury-autism connection appear trivial.
We can start by comparing a 200-pound male adult consuming tuna with the infant who receives a single vaccine on their first day of birth (since day-old infants don't eat tuna). On the first day of birth an infant receives the Hep B vaccine with about 25 micrograms of ethlymercury - this does approximate the 30 micrograms of methlymercury in an average can of tuna. Since the average infant weighs about 7 pounds, the weight equivalent number of cans of tuna for an adult would be 28 cans. (The adult male weighs 28x more than the infant.)
If you take those 28 cans of tuna and distill it down to mercury content, you would have 840 micrograms of mercury (30 micrograms per can). Keep in mind that the stomach successfully absorbs and excretes about 90% of any mercury ingested through food, leaving only about 10% of the mercury to be absorbed into the bloodstream. Since the mercury in vaccines is injected directly into the bloodstream where 100% of it can be absorbed by the organs, you would need an additional 252 cans of tuna to get the equivalent amount of mercury into the bloodstream for a total of 280 cans of tuna and 8,400 micrograms of methlymercury.
Also, remember that a developing brain is far more sensitive to toxins than an adult brain. Current estimates say mercury is 5-10x more toxic for a developing brain. We'll use the low end of that range, so multiply the 280 cans of tuna by 5 and you get 1,400 cans of tuna.
So, receiving the Hep B vaccine with Thimerosal on the first day of birth is the equivalent of a 200-pound adult male consuming 1,400 cans of tuna in a single day. One final adjustment: the adult male in the analogy needs to have no capacity to excrete mercury. As Boyd Haley, Ph.D. notes, "it is very well known that infants do not produce significant levels of bile or have adult renal capacity for several months after birth. Bilary transport is the major biochemical route by which mercury is removed from the body, and infants cannot do this very well."
So, a 200-pound male who consumes 1,400 cans of tuna in a single day and has their ability to excrete mercury severely diminished is the same as a day-old infant receiving the Hep B vaccine. Now the analogy is fair.