Autism, mercury, and vaccinations

Two science bloggers I respect, Pharyngula and Orac, say the idea that mercury in vaccinations causes autism is garbage.

I'm unconvinced for either side. Orac's best point is that mercury was removed 10 years ago from vaccines in Canada and Denmark, and he says rates haven't plunged there. However, he doesn't cite any studies that tried to carefully analyze whether the removal made any difference. Certainly the removal didn't cause rates to plunge, that would be obvious, but if it caused a minor decrease in incidence of autism and maybe a less-minor decrease in severity, we might not know it without careful studies.

Orac says the massive increase in autism is an increase in diagnosis, not an increase in incidence. That concept is highly controversial, as you can see just from clicking his links. I need further consensus before I'll agree that either side is right.

All in all, this is an interesting test case of the precautionary principle. I don't believe that any imagined connection to a harm is sufficient to invoke the precautionary principle and stop an action from occurring until it has been proven safe. The question is how much evidence do you need of how severe a harm before you invoke the principle. Mercury in vaccines is right on the edge of sufficient evidence I think, and it'll be interesting to watch the evidence develop in one direction or another.

UPDATE: more shoes drop: Majikthise, another good science blog, doubts the mercury-vaccine-autism connection. Some well-written comments to the post suggest otherwise though, including the statement that Canada and Denmark had used very little mercury in vaccines prior to 1995 anyway (no source given for the claim, however). Wampum, another good blog, weighs in on the other side. Debate will have to go on...

UPDATE 2: Majikthise has yet another good post on the thimerosal issue. Even though she's doubtful, her summary lays out grounds for concern IMHO. Nosenada notes the conflict-of-interest issue that's been left out of the discussion (and he links to me here - obviously that's an important post!)

UPDATE 3: NYTimes weighs in, clearly favoring the "no connection to autism" side. Most relevant paragraph:

"In one of the most comprehensive studies, a 2003 report by C.D.C. scientists examined the medical records of more than 125,000 children born in the United States from 1991 to 1999. It found no difference in autism rates among children exposed to various amounts of thimerosal."

I think it's clear that thimerosal is not THE sole cause of increased autism, and that researchers like Dr. Geier (criticized in the article and elsewhere) have no credibility. I don't think that's the end of the matter though, especially where exposure may affect other developmental disabilities, which could be evidence of a slight effect on autism.

keywords: science, mercury, precautionary principle