[link|http://www.fda.gov/cber/vaccine/thimfaq.htm|Thimerosal FAQ].

Specifically:

Autism and autism-spectrum disorders have been steadily increasing, especially during the 80\ufffds and 90\ufffds. During this time period the number of vaccines that children have received has more than tripled. Doesn\ufffdt this implicate vaccinations as a cause of autism?

The reasons for the apparent increase in the number of cases of autism over the past two decades are complex. In part, the increase can be traced to a broadening of the case definition to include less severe and more atypical presentations of autism. However, the increased number of childhood vaccinations and increased vaccine coverage in recent years in no way constitutes evidence of an association with autism or any other diseases which may have increased in recent years. To the contrary, childhood vaccinations today protect children from devastating illnesses such as meningitis. (The rubella vaccine administered in infancy protects the fetus of the next generation from neurological deficits, and may arguably be described as an anti-autism vaccine).

The association that has been noted by some concerned parents of autistic children that the increase in the prevalence of autism over the last few decades \ufffdclosely matches the introduction and spread of thimerosal-containing vaccines\ufffd is known as an ecological study. Ecological studies alone are generally not accepted as strong evidence of causality, because they do not link individual exposure to individual outcome, and can be subject to confounding by unknown or uncontrollable factors. In addition, it has been noted that some children with autism have high levels of mercury in hair, urine and blood. This observation cannot be interpreted without information on the levels of mercury in individuals without autism (i.e. case-control study). However, such observations do suggest that the hypothesis should be studied further.

The FAQ addresses many of the questions that have come up here (but often not in a great deal of detail). More details on thimerosal are [link|http://www.fda.gov/cber/vaccine/thimerosal.htm|here].

Note that the measles, mumps and rubella (MMR) vaccine never contained thimerosal. I'm sure that many recall that in the 1990s there were worries that the [link|http://www.vaccineinformation.org/measles/qandavax.asp|MMR vaccine caused autism]:

Does the MMR vaccine cause autism?
Current scientific evidence does not support the hypothesis that measles-mumps-rubella (MMR) vaccine causes autism. The question about a possible link between MMR vaccine and autism has been extensively reviewed by independent groups of experts in the U.S. including the National Academy of Sciences' Institute of Medicine. These reviews have concluded that the available epidemiologic evidence does not support a causal link between MMR vaccine and autism.

The MMR-autism theory had its origins in research by Andrew Wakefield and colleagues in England. They suggested that inflammatory bowel disease (IBD) is linked to persistent viral infection. In 1993, Wakefield and colleagues reported isolating measles virus in the intestinal tissue of persons with IBD. The validity of this finding was later called into question when it could not be reproduced by other researchers.

The studies that suggest a cause-and-effect relationship exists between MMR vaccine and autism have received a lot of attention by the media. However, these studies have significant weaknesses and are far outweighed by many population studies that have consistently failed to show a causal relationship between MMR vaccine and autism.

For a summary of the issues on this topic, please read "Vaccines and Autism," by Paul A. Offit, MD, Director, Vaccine Education Center, Children's Hospital of Philadelphia. This article can be accessed online at: [link|http://www.immunize.org/catg.d/p2065.htm|www.immunize.org/catg.d/p2065.htm]

I'm sure that some people who are now pointing toward thimerosal used to point to the MMR vaccine. They simply don't trust that the vaccines are safe and desperately want to find a cause of their child's Autism. Hell, I'd like to know what caused it in my brother too. I suspect, though, that it has a very strong genetic component.

Since symptoms of Autism typically appear a few months after some immunizations, it's natural to think that there's a link to vaccines. But since the brain develops very slowly - in fact it is continuing to mature [link|http://www.abanet.org/crimjust/juvjus/Adolescence.pdf|into the early 20s] - it's not hard to recognize that Autism might be set in motion in the womb and only appear when the brain has achieved a certain configuration.

It's a complicated problem and "obvious" associations often aren't correct. That's why it's important to do careful, controlled studies of large groups to have some hope of figuring out what's really going on in situations like this.

Cheers,
Scott.