There are a few possible answers: celebrity backing, scientists not effectively dispelling inaccuracies, or just people hearing the message so many times that it gains a veneer of truth. All of these have some merit, and all are involved.
But there are two other ideas I want to throw into the pile as well, the first being;
What if psychological biases influence us away from fully rejecting Wakefield’s story, despite the facts?
In politics, sports, and talent shows, describing one option as the underdog is an easy way to encourage support for them. Research on sports fandom shows that people usually support the team defined as underdogs. Yet if the underdogs start winning, people often switch allegiance to the newly-losing team.
People’s opinion on events as large as the Israel-Palestine conflict could be influenced simply by changing how an explanation of the conflict was illustrated. 75% of people supported Israel when it was shown as a tiny area surrounded by the rest of the Middle East: only 45% of people supported Israel when Israel was shown to dwarf a surrounded Palestine. Ultimately, we are biased towards supporting underdogs in many cases, especially when we see the underdog as similar to ourselves. (However its opposite, the bandwagon effect, can also influence political and social decisions).
Many anti-vaccine websites gain sympathy and attention by portraying Andrew Wakefield as the perennial underdog, and painting all actions against him as victimisation. They can also show anti-vaccination proponents as underdogs; proud parents who are being prevented from doing what is best for their children. Given that characterisation, their arguments are harder to silence; to take away the voice of an underdog has a note of cruelty that a lot of people won’t want to confront. In an in-depth Slate article on the underdog bias, the writer proclaims “rooting for the little guy is the American way”. This brings me on to the next idea:
What if the plots and patterns of similar fictional situations have predisposed us towards one conclusion?
In media, agenda-setting theory is the idea that news coverage can decide which topics people recall and consider important. Heavily-covered topics and ideas can be recalled more easily and therefore seen as more important. This can also extend to interpersonal communication – easily-recalled topics are also more easily passed on and discussed between people. If ideas can be passed on between people as conversational currency, becoming part of people’s mental libraries and defaults, what if fictional topics have a similar effect?
In films where a large organisation squares up to a single dissenter, the organisation is usually a corrupt silencer of the dissenter’s uncomfortable truth. A happy ending means the little guy wins, restoring his dignity and derailing the organisation.
If this story was a film, Andrew Wakefield would be the discoverer of a sinister truth, and everything after his paper- from his research being retracted, to being struck off the medical register, to the campaigns promoting vaccination in response- would be the actions of a massive interleaving conspiracy preventing the truth escaping at any cost. This view resembles how many anti-vaccination proponents view the story.
So, what if that view has been reinforced because we have spent our lives watching that archetypal plot play out, and so this story has generated massive cognitive dissonance? What happens when the story we expect is reversed; “Big Pharma” is the hero, while the little guy is the villain? Maybe this controversy has prevailed so long, and has been difficult to silence, at least partly because no-one has a script in place for what to do in this situation.