Two fears powered the unfolding of these events. One was a fear of the unfamiliar ingredients in vaccines, including the mercury-containing preservative thimerosal, which as of mid-2014 was still used in some multidose flu shots but was otherwise phased out of routine childhood vaccines in the United States starting in 2001. The other was a fear of autism, an anxiety fostered by media stories pitting emotional appeals by high-profile anti-vaccine advocates against statistically based reports by medical researchers.
These fears persisted even as evidence mounted that they were completely unfounded. Scientific verification relies on a process of testing and confirmation, not on a single observation. Researchers sincerely grappled with the question of a vaccine-autism link in numerous studies following publication of the 1998 Lancet paper. Some of these studies analyzed data from millions of people, in the quest to see if vaccines and autism might be linked. The overwhelming scientific consensus is that they are not. The Lancet paper specifically addressed a possible association between the MMR vaccine and autism, but later studies also looked at other vaccine-related factors, such as the mercury-containing preservative thimerosal, which is not used in the MMR vaccine. Thimerosal initially raised concerns because mercury is neurotoxic. However, no link with autism—not to mercury, not to thimerosal, not to any vaccines, including the MMR—has been found.
In some fraction of the American population, however, the belief in a link remains. One reason is a coincidence of timing: children are routinely vaccinated just as parents begin to observe signs of autism. Most vaccines are administered during the first years of life, which is also a period of rapid developmental changes. Many developmental conditions, including autism, don't become apparent until a child misses a milestone or loses an early skill, a change that in some cases can't help but be coincident with a recent vaccination.
Adding to such concern is the fact that, sometimes, vaccinations can lead a child to develop a high fever and accompanying febrile seizures. Such seizures are temperature-related and don't cause lasting damage. A tendency to experience febrile seizures runs in families, and about one in 20 young children will have one at some point. According to the U.S. Centers for Disease Control and Prevention, most febrile seizures happen when a child is sick rather than after a vaccination, though the MMR vaccine is associated with a slightly increased risk for febrile seizures. Children with and without autism have these febrile responses, but since their timing may coincide with emerging signs of autism, that can link the two incidents in a parent's mind, even though there is no causal relationship