Many proponents of the belief that vaccines cause autism mix elements of variants of this theory in illogical ways. For instance, regressive autism was originally tied to the MMR vaccine largely because of the temporal association between the first MMR shot between 14 and 18 months and the onset of regressive autism in some children. There is at least some logic to making this connection for parents who believed they saw autism symptoms developed shortly after the MMR shot.

However, when thimerosal took off as a theory of autism in America, it was also tied to regressive autism. But, there is no logic at all to that connection, largely because the MMR vaccine (which is the vaccine that occurs around the same time as regression is frequently reported) does not contain thimerosal and never has. Before the removal of thimerosal from vaccines in the US, the vast majority of thimerosal exposure occurred during the first 7 months of life, when seven thimerosal containing vaccinations were given (one shot for Hep B shot and three shots each for DPT and Hib B). The ethyl mercury in thimerosal has a very short half life, meaning most ethyl mercury from thimerosal is gone within a couple weeks or so of the shot, preventing the type of accumulation of heavy metals that can be toxic. There is no logical reason why an exposure to a heavy metal in the first few months of should trigger a quick regression into autism 12 to 18 months later, since all of the mercury from the vaccines would have cleared the system 10+ months before.

The same logical problem exists for those who try to use regressive autism to support a theory that it is the total vaccine load that is causing autism, which is increasingly occurring today since such strong evidence has accumulated against thimerosal involvement in autism. Children in the US are given most of their vaccinations in the first 7 months of life and these are spaced relatively far apart. Then, only a few are given thereafter. No one has explained why vaccinations, acting through general immune response activation caused by an increasing total vaccine load given during the first months of life, are logically tied to autism that develops suddenly a year or two later.