So... questions from the audience... How much Mercury are we talking about in the vaccine? How often do you get a flu shot, or any shot, with thimerosal in it? And over how many years will it take for those shots to equal .... say... the amount of mercury in a tuna salad submarine sandwich? And then, how much of it will it take to do anything to you? And finally, does mercury exposure cause Alzheimer's?
Long answers below- Short answers here: The mercury in one shot is almost too small to be measured. One shot, once a year, will most likely NOT increase your exposure in any significant amount. And there is no direct evidence that there is any causal relationship between mercury and Alzheimer's.
And now the long answers from the people that don't send out panic emails.
There's no evidence that influenza vaccine increases the risk of Alzheimer's disease. Inactivated influenza vaccine used in the United States may contain thimerosal, a preservative that contains a very small amount of mercury. There's no evidence that the mercury in thimerosal is harmful at these low levels.
Alzheimer's is a complex illness involving damage to and degeneration of the neurons within the brain. Its cause isn't fully understood. But it may be due to abnormal protein (amyloid) deposits and inflammation in the brain. It was once thought that aluminum contributed to this process. But most experts believe that there's no clear evidence to support this theory. Influenza vaccine doesn't contain aluminum.
And then we have a statement from the resource page from the Center's for Disease Control.
Today, with the exception of some Influenza (flu) vaccines, none of the vaccines used in the U.S. to protect preschool children against 12 infectious diseases contain thimerosal as a preservative.