This is a very important question in light of a recent study undertaken by Brendan Nyhan and colleagues. They randomly contacted Americans by phone to participate in an on-line examination of the effects of 4 different kinds of messages on subjects' (1) self reported attitudes towards MMR vaccine and (2) their propensity to use the vaccine in the future. Although a message correcting misinformation about the MMR vaccine causing autism changed respondents' minds about that particular concern, none of the interventions increased the respondents propensity to use the vaccine in the future; they actually decreased it.
What will change the minds of anti-vaxxers?
I suggest that we ask them that question.
Anti-vaxxers should be asked to consider the following scenario: imagine that we live in a world where the MMR vaccine is 99% effective at preventing MMR infection with only a 1 in a million risk of a serious adverse event, like a life-threatening allergic reaction. That's it - no risk of the vaccine causing any other adverse effects. Period. And we know this for a fact. Close your eyes, take a deep breath, and as you exhale, let the imaginary scenario where that is perfectly true sink deeply in.
Anti-vaxxers should have to tell the rest of us what it would take to convince them that that is indeed true. Remember, for the sake of argument, it is true. The question is what would be required for them to recognize and acknowledge that it is.
Here's the catch: children would still be developing features and diagnoses of autism, MS, hepatitis, arthritis, learning and behavioral disabilities, and a host of other mysterious disorders some time after getting vaccinated, even though none of those conditions would, in our imaginary scenario, be causally related to the MMR vaccine. What would it take to convince them that that was the case despite those coincident occurrences happening to kids and families everywhere?
That is the crux, isn't it? The truth is that we simply cannot trust our personal experiences (and those of our friends and family as well) to draw any conclusions about events like these. They are completely unreliable. Only population based studies and large randomized trials can reliably inform us about potential causal correlations like these. Asking anti-vaxxers this question forces them to struggle with that fact. In the end, I think that there are only 2 possible kinds of responses.
First, some would realize that nothing could convince them of the truth of such a scenario. These people may never change their minds - they aren't open to having their minds changed. Unfortunately, this stance indicates a complete unwillingness to even try to recognize a real world scenario that might actually be true and that would actually be wonderful. All too common, stances such as these - the definition of irrationality - are dangerous conversation stoppers. Perhaps peer-pressure, mockery, satire, and ridicule would work, but no amount of evidence would seem likely to satisfy.
The second response would have to be some kind of evidence. What else?
And so it would seem that if there are anti-vaxxers that are open to having their minds changed (and there are), despite Nyhan's study results, the thing that will do the trick must be evidence. That's a very good thing, because as long as the conversation is one about evidence, there is a conversation to be had. The key will be determining what kind of evidence may be required and how to best present it so as to have have the greatest impact. Nyhan's important work remind us that we should study the effects of our efforts to change minds very carefully. I suggest that anti-vaxxers themselves should be the ones telling us where to start.
You know that imaginary scenario I described above? It's actually pretty close to the truth.