Models from epidemiology are increasingly used to better understand how misinformation spreads in online networks.1 If misinformation behaves like a virus, invading a susceptible host and rapidly spreading from one mind to another,2 then the development of a psychological vaccine seems essential. Luckily, there is a science of mental immunity and there’s a lot to like about the analogy at the heart of much mental immunity research: inoculation theory. Inoculation theory’s analogic namesake tells us what inoculation theory does and explains a good deal of how it’s doing it, and we think it’s doing it well. The analogy is of value to scholars beginning to do work with this theory as a heuristic and as a primer, and it’s of value to the most senior inoculation scholars as a springboard for theoretical advancement and innovative applications. And it’s of value in these ways for many of the same reasons: it’s clear, it’s accurate, and it’s inspiring.
So let’s think a bit more about the analogy of inoculation theory—how medical inoculation against viruses can parallel inoculation against (misleading) arguments, and let’s also consider the analogy and inoculation theory—what we learn about inoculation theory by studying its moniker, and what we learn about analogic reasoning by keeping it in mind when we do inoculation theory work.
But first, what do we mean by the inoculation theory of resistance to influence? Inoculation theory—as a named, formal theory—has been around since the early 1960s, when the social psychologist William McGuire offered it as an explanation for how a belief could be made more resistant to persuasive efforts to change that belief,3 which he termed "a vaccine for brainwash”. At the time, McGuire was concerned about narratives that American soldiers were being “brainwashed” by enemy troops during the Korean War. He reasoned that the real issue was that the soldiers had no “mental defenses” against the types of ideological attacks they would be facing. Of course, as a concept, it goes back much farther than the 1960s, including Aristotle’s ‘refutational enthymeme’, or the idea that one could preempt an argument by getting ahead of it and making one’s case beforehand.4
But it was McGuire who first leaned so heavily on analogic to explain how and under what conditions a belief could be made more resistant to change, and the vaccine analogy does a nice job explaining it. Just as a body can be made more resistant to viral threats through preexposure to weakened forms of the virus, a belief can be made more resistant to persuasive threats through preexposure to weakened forms of the persuasion.
In a medical vaccine, a virus might be weakened by being inactivated or killed, leaving enough of the appearance of a threat to our body’s immune system to motivate protective measures, like the production of antibodies. In a cognitive vaccine, a counterargument might be weakened through preemptive refutation or by exposing the manipulation strategies at work. Like with medical vaccines, the idea is to leave enough of the persuasive attempt to be concerning (called threat) so that it motivates protective measures, like the production of supporting arguments to shore up the belief. So, in a way, just like biological antibodies can be detected in our blood, psychological antibodies can be concretely measured and observed, too. For example, by the number of counter-arguments people can generate against a false claim, or the increased motivation they feel to avoid being duped or manipulated. We can even see changes in people’s memory networks where the number of concepts and the connections between relevant concepts are strengthened after receiving an inoculation.
One of the remarkable things about psychological inoculation is that its efficacy does not seem limited to specific content included in an inoculation message—the specific counterarguments that are raised and preempted. Just as some vaccines prove effective against multiple strains of the same virus, raising and refuting a handful of potential counterarguments protects against many counterarguments.5 This seems to especially be the case when designing inoculation messages that protect against manipulation strategies used in the spreading of misinformation—strategies that can be attempted across a range of issues and a range of contexts.6 For example, once people are inoculated against the core building blocks of a conspiracy theory, they are more likely to resist being persuaded by a whole range of specific conspiracy theories whether it’s about climate change or COVID-19.
Indeed, over the years, inoculation theory has established itself as a powerful messaging strategy across a range of issues and a range of contexts.7 Inoculation campaigns have successfully combatted mis- and disinformation about climate change,8 vaccinations, social and political policies,9 peer pressures to engage in risky behaviors,10 and more. It’s been used to confer resistance to manipulation and propaganda strategies in general,11 and it’s been used to shore up confidence in one’s abilities before stressful events like public speaking.12
More recently, inoculation scholars have discovered that the process of cognitive inoculation not only models prophylactic, or preemptive, medical inoculation, but also seems to mirror recent advancements in therapeutic medical inoculation. That is, just as medical vaccinations on HPV and HIV show promise for healing benefits—healing those already sick and conferring resistance to future viral attacks—psychological vaccinations show promise for healing benefits—changing the minds of those already misled, protecting those who may have been ‘exposed’ but not yet ‘infected’ and conferring resistance to future persuasive attacks.13
But the analogy still extends further. Our biological immune system contains so-called memory T-cells, which recognize specific antigens. They can remain in the body long after an infection has passed so that when the body is re-exposed to the virus, they can provide a quick immune response. However, the initial flood of antibodies we receive after vaccination, the so-called ‘frontline fighters’, naturally wear off over time. Similarly, research finds that the effectiveness of cognitive inoculations also wear off over time; people sometimes forget what they have learned or they lose motivation to keep defending themselves. Accordingly, just as some biological vaccines require ‘booster shots’, research on cognitive vaccines has shown that booster shots are essential for ensuring the long-term efficacy of the inoculation process.14
In the final act of the analogy, we have to recognize the end game of individual vaccinations, namely: herd immunity. When enough people are vaccinated, the virus no longer has a chance to spread throughout the population. When it comes to cognitive inoculation, scholars have discovered the process of ‘post-inoculation talk’, i.e., people engage in ‘talk’ after receiving a psychological inoculation. This process can not only strengthen the inoculation itself but also cause the inoculation to spread from one person to another via word-of-mouth15 or even online via social networks.16 Ultimately, if a sufficient number of people are made cognitively immune, conspiracy theories, partisan rhetoric, and misleading information will no longer be able to spread. The beauty of herd immunity is that not everyone can or needs to be vaccinated: the herd is protected so long as we reach a sufficient audience.
There’s still a lot to work out about this theory, but here’s what we’re thinking now: We gain a lot by keeping the analogy of inoculation theory in mind as we work toward building better inoculation messaging, application, and theorizing.
 van der Linden, S. (2023). Foolproof: Why Misinformation Infects Our Minds and How to Build Immunity. New York, NY: WW Norton. ISBN: 978-0-393-88144-8.
 McGuire, W. J. (1964). Inducing resistance to persuasion: Some contemporary approaches. In L. Berkowitz (Ed.), Advances in experimental social psychology 1, (pp. 191-229). New York: Academic Press. https://doi.org/10.1016/S0065-2601(08)60052-0
 Compton, J. (2005). Tracing the roots of resistance to influence: Comparison, contrast, and synthesis of Aristotelian rationality and inoculation. STAM Journal, 35, 1-23.
 Traberg, C. S., Roozenbeek, J., & van der Linden, S. (2022). Psychological inoculation against misinformation: Current evidence and future directions. The ANNALS of the American Academy of Political and Social Science, 700(1), 136-151. https://doi.org/10.1177/0002716222108793
 Ivanov, B., Parker, K. A., & Dillingham, L. L. (2020). Inoculation theory as a strategic tool. In H. D. O’Hair and M. J. O’Hair (Eds.), Handbook of Applied Communication Research (Vol. 1, pp. 13-28). Wiley.
 van der Linden, S., Leiserowitz, A., Rosenthal, S., & Maibach, E. (2017). Inoculating the public against misinformation about climate change. Global Challenges, 1(2), 1600008. https://doi.org/10.1002/gch2.201600008
 Compton, J., & Ivanov, B. (2013). Vaccinating voters: New directions for political campaign inoculation scholarship. Annals of the International Communication Association, 37 (1), 250-283. http://dx.doi.org/10.1080/23808985.2013.11679152
 Parker, K. A., Ivanov, B., & Compton, J. (2012). Inoculation’s efficacy with young adults’ risky behaviors: Can inoculation confer cross-protection over related but untreated issues? Health Communication, 27 (3), 223-233. https://doi.org/10.1080/10410236.2011.575541
 Roozenbeek, J., van der Linden, S., Goldberg, B., Rathje, S., & Lewandowsky, S. (2022). Psychological inoculation improves resilience against misinformation on social media. Science Advances, 8(34), eabo6254. https://doi.org/10.1126/sciadv.abo6254
 Jackson, B., Compton, J., Thornton, A. L., & Dimmock, J. A. (2017). Re-thinking anxiety: Using inoculation messages to reduce and reinterpret public speaking fears. PLoS ONE 12(1): e0169972
 Maertens, R., Roozenbeek, J., Basol, M., & van der Linden, S. (2021). Long-term effectiveness of inoculation against misinformation: Three longitudinal experiments. Journal of Experimental Psychology: Applied, 27(1), 1-16. https://doi.org/10.1037/xap0000315
 Compton, J., & Pfau, M. (2009). Spreading inoculation: Inoculation, resistance to influence, and word-of-mouth communication. Communication Theory, 19, 9-28. https://doi.org/10.1111/j.1468-2885.2008.01330.x
 Pilditch, T. D., Roozenbeek, J., Madsen, J. K., & van der Linden, S. (2022). Psychological inoculation can reduce susceptibility to misinformation in large rational agent networks. Royal Society Open Science, 9(8), 211953. https://doi.org/10.1098/rsos.211953