Some vaccine websites state that because we injest high levels of aluminum, the injected aluminum is no big deal. Well, consider this quote from Dr. Sears:
"We know aluminum is a neurotoxin. We also know that humans can ingest huge amounts without harm, since 99% of it passes out through the stools. I’m sure Dr. Offit knows that, so I’m curious as to why he’d use the “babies ingest tons of aluminum anyway” argument." 
So only 1 percent of ingested aluminum is absorbed.
Now let's look at the amount of aluminum babies consume:
"Breast milk contains 40mcg/L aluminum, milk-based formulas contain ~225mcg/L, and soy-based formulas contain ~500mcg/L."
1% of each of those amounts equals .4 mcg, 2.25 mcg, and 5 mcg. So their kidneys only have to filter about .4 to 5mcg of aluminum from daily milk/formula consumption.
However, 100% of injected aluminum has to be filtered by the kidneys. So, the amount of aluminum in vaccines is not just 'a drop in the bucket' compared to consumed aluminum.
.4 mcg - 5 mcg of aluminum from food sources pales in comparison to the 295 - 1225 mcg a child could receive in one day from vaccines (The exact amount depends on the brands given).
A very recent study (Here) found that at least 100 mcg of daily aluminum consumption was associated with greater cognitive decline. 1% of 100 mcg of ingested aluminum is 1 mcg, which would imply that breast milk at .4 mcg is perfectly safe, and perhaps soy infant formula isn't such a great idea.
And furthermore, in the book, "Aluminum and Health" By Hillel J. Gitelman, the small amounts of aluminum absorbed by the intestines are usually excreted by the kidneys, and accumulation only occurs in cases of high chronic intake. However, the aluminum in vaccines may not be as easily excreted. With the intention of defending vaccine safety, Dr. Paul Offit said that after 2 weeks, 85% of the aluminum from vaccines has been excreted (and that's for the average healthy person). But, unfortunately, that would mean that we still have 15% of 295 mcg - 1225 mcg of aluminum unaccounted for (equalling 44.25 mcg - 183.75 mcg from just one office visit). Has anyone done any studies to see what happens to the remaining 15% of the injected aluminum? Is it ever excreted, or does it accumulate in brain and skeletal tissue?
I added the above information after reading the study about aluminum consumption.
Here is my old post:
The toxic dose of aluminum for an adult is 350 mcg or .350 mg. A certain brand of OTC extra strength antacid tablets contains 160 mg of aluminum hydroxide in each tablet, and the directions say to take 2-4 tablets up to four times a day. So there has to be a difference between ingested and injected aluminum or else a single antacid tablet would be toxic.
So, the high levels of aluminum found in soy baby formula cannot be used to say that the aluminum in vaccines is safe. Also, already being exposed to a toxic substance puts you at a higher risk of toxicity, not a lower risk.
And really, it just makes sense that ingested would be different than injected. If a child swallows a dime, the whole dime is passed in the stools. It is not digested, and therefore does not enter the bloodstream. However, vaccines are injected into muscle tissue and 100% of the ingredients are absorbed, over time, and therefore have to be filtered out by the kidneys.
So why does this matter? Click on this link to read an excerpt of Dr. Sears' book that shows how high the aluminum levels are and the risks this causes: http://www.mothering.com/articles/growing_child/vaccines/aluminum-new-thimerosal.html
The Vaccine Book by Dr. Bob Sears, contains much more information about aluminum toxicity studies and cites, among other things, that aluminum can cause neurological harm, newborns are at increased risk of aluminum toxicity, aluminum toxicity is NOT rare in newborns, and that this toxicity is difficult to detect by observing symptoms.