The autism cases that receive the most public attention and that elicit the greatest amount of parental fear are the regressive cases, typically where pre-existing language skills are lost or reduced between 12 and 24 months. It is also the type of autism that fits best with the vaccine theory of autism, particularly the variant that says the first MMR vaccination around 14-18 months causes autism. The reported temporal association between vaccination and autism onset in certain cases of regressive autism is exploited by those seeking to implicate vaccines in autism.

Based upon media attention and internet traffic, it would seem that regressive autism is the dominant form of the disorder. However, that is not the case. Reported rates of regressive autism vary, but most estimates are in line with one study which showed regression in 23% of atypical autism cases and 27% of those with childhood autism. The typical autism case involves autistic abnormalities developing early in life with a consistent delay in meeting developmental milestones and no regression such as loss of language. This typical onset is a disorder progression that is not consistent with the vaccine theory of autism, where one would expect a reaction to a particular vaccination such as fever or seizures followed by regression.

Another important factor in this analysis is that the lower the severity of autism, the less frequent regression is. For instance, one study reported regression in only two percent of the children with Asperger’s Syndrome. This fact does not make logical sense under a vaccine theory. If autism is caused by vaccines, and regression is a sign of vaccine induced autism (as seems to be the argument), then regression should be typical in both low and high functioning variants of autism. There is no logical reason why vaccines should only cause certain forms of autism, as evidenced by regression only happening commonly in certain forms of autism.