Unfortunately, an anti-vaxxer is really just a conspiracy theorist. Some of them even admit they would not change their mind in light of any evidence, or keep shifting the goalposts until the proof they demand is impossible to provide.

This is the crux of the issue: you can’t tackle a conspiracy theorist with facts or evidence, because their logic prevents them from taking into account anything that contradicts their worldview.

In fact, Lewandowski and his colleagues have outlined six criteria for conspiratorial thinking, and most of those apply to anti-vaxxers. Allow me to demonstrate with examples gleaned on anti-vaccination forums online.

The first criterion is nefarious intent — the belief that presumed conspirators have evil intentions. For anti-vaxxers this usually goes along the lines of Big Pharma “killing babies for money.” Similar narratives include careless doctors, incompetent health authorities, research fraud and other explanations for why nobody believes that vaccines are bad. In other words, every health official is in on it and the people who don’t see this are fools. Obviously.

Then there is the perception of oneself as a persecuted victim. Sure enough, anti-vaxxers are usually quick to complain about suppression of “the truth” they have discovered, particularly when health authorities issue warnings about misleading anti-vaccination campaigns.

The next criterion that fits the bill is nihilistic skepticism, the attitude anti-vaxxers show when they refuse to believe in anything that doesn’t fit their theory.

And lastly, self-sealing reasoning means that any evidence against the conspiracy is interpreted as evidence for it. This is exactly what anti-vaxxers do when they face a study like Eslick’s: they claim the data has been cherry-picked, the author has vested interests, the University of Sydney received new lab equipment for running this research, and so on and so forth.

You can’t debate a conspiracy theorist using evidence and reason. It’s absolutely crucial to realise this when faced with claims by anti-vaccination campaigners.

Anti-vaxxers are immune to evidence.

And you know what — it’s really disappointing. We need watchdogs for the medical industry as much as we need them everywhere else. Vaccines are not without risks, but they prevent diseases. And yes, pharmaceutical companies are known to engage in shoddy and sometimes unethical practices. Manufacturing mistakes happen, and people can be harmed by medications.

But it is crucial that the discussion of these issues is done in a rational environment that encourages real debate and bases arguments on solid evidence. Not with militant activists who refuse to accept anything that won’t fit their narrative.

Scientists make mistakes and correct them, but you can’t expect a conspiracy theorist to admit they are wrong.