Inoculation Theory was developed by social psychologist William J. McGuire in 1961 to explain more about how attitudes and beliefs change, and more importantly, how to keep original attitudes and beliefs consistent in the face of persuasion attempts. Inoculation Theory continues to be studied today by communication, social psychology, and social science researchers. The theory has been assessed in varied context, including politics (Pfau et al., 1990), health campaigns (Pfau & VanBockern, 1994), and marketing (Compton & Pfau, 2004), among others.
Medical analogy 
Inoculation can best be explained by a medical inoculation analogy. Indeed, the analogy served as the inaugural exemplar for how inoculation confers resistance. As McGuire (1961a) initially explained, a medical inoculation works by exposing a body to weakened viruses—strong enough to trigger a response (i.e., the production of antibodies), but not so strong as to overwhelm the body's resistance. Attitudinal inoculation seems to work the same way: Expose someone to weakened counterarguments, triggering a process of counterarguing which eventually confers resistance to later, stronger persuasive messages.
The idea of inoculation was derived from previous research studying one-sided and two-sided messages. One-sided messages are supportive messages to strengthen existing attitudes, but with no mention of counterpositions. Two-sided messages present both counterarguments and refutations of those counterarguments (Lumsdaine & Janis, 1953).
One of the greatest motivators for McGuire was the aftermath of the Korean War. Nine US prisoners of war, when given the opportunity, elected to remain with their previous captors. Many assumed they were brainwashed, so McGuire and other social scientists turned to ways of conferring resistance to persuasion. This was a change in extant persuasion research, which was almost exclusively concerned with how to make messages more persuasive, and not the other way around (Gass & Seiter, 2003).
McGuire led a series of experiments assessing inoculation's efficacy and adding nuance to our understanding for how it works (for a review, see Compton & Pfau, 2005). The early studies (e.g., McGuire & Papageorgis, 1961) limited testing of inoculation theory to cultural truisms, or beliefs accepted without consideration (e.g., people should brush their teeth daily.) Later development of the theory extended inoculation to more controversial and contested topics in the contexts of politics, health, marketing, and others. The theory has also been applied in education to help prevent substance abuse. 
Explanation of the theory 
Inoculation theory states that to prevent persuasion it is necessary to strengthen preexisting attitudes, beliefs, or opinions. First, the receiver must be warned of an impending attack. This establishes threat (or a recognition of vulnerability) and initiates defenses to future attacks. Therefore, the idea is that when a weak argument is presented in the inoculation message their process of refutation will prepare for stronger persuasion later. It is critical that the attack is strong enough to keep the receiver defensive, but weak enough to not actually change those preexisting ideas. This will hopefully make the receiver actively defensive and allow them to create arguments in favor of their preexisting thoughts. The more active the receiver becomes in his or her defense the more it will strengthen their own attitudes, beliefs, or opinions (McGuire, 1964).
Key components 
There are two basic key components to successful inoculation. The first is threat, which provides motivation to protect one's attitudes or beliefs (Pfau, 1997a). Refutational preemption is the second component. Refutational preemption is the cognitive part of the process. It is the ability to activate one's own argument for future defense and strengthen their existing attitudes through counterarguing. (Pfau, 1997)
Refutational same & refutational different 
While there are many studies that have been conducted comparing different treatments of inoculation, there is one specific comparison that is mentioned throughout various studies. This is the comparison between what is known as refutational same and refutational different messages. A refutational same message is an inoculation treatment that refutes specific potential counterarguments that will appear in the subsequent persuasion message, while refutational different treatments are refutations that are not the same as those present in the impending persuasive message (Pfau et al., 1990). Pfau and his colleagues (1990) developed a study during the 1988 United States presidential election. The Republicans were claiming that the Democratic candidate was known to be lenient when it came to the issue of crime. The researchers developed a refutational same message that stated that while the Democratic candidate was in favor of tough sentences, merely tough sentences could not reduce crime. The refutational different message expanded on the candidate's platform and his immediate goals if he were to be elected. The study showed comparable results between the two different treatments. Importantly, as McGuire and others had found previously, inoculation was able to confer resistance to arguments that were not specifically mentioned in the inoculation message.
Relevant research 
Political Campaigning 
One study, previously mentioned, was conducted by Pfau and some of his colleagues examining inoculation through the use of direct mail during the 1988 presidential campaign. The researchers were specifically interested in comparing inoculation and post hoc refutation. Post hoc refutation is another form of building resistance to arguments, however, instead of building resistance prior to future arguments, like inoculation, it attempts to restore original beliefs and attitudes after the counterarguments have been made. Results of the research reinforced prior conclusions that refutational same and different treatments both increase resistance to attacks. More important, results also indicated inoculation was superior to post hoc refutation when attempting to protect original beliefs and attitudes. (Pfau et al., 1990)
Smoking and Young Children 
Another study conducted by Pfau and his colleagues (1992) examined the role of inoculation when attempting to prevent adolescents from smoking. One of the main goals of the study was to examine longevity and persistence of inoculation. They took a group of elementary school students in South Dakota and had the students watch a video inoculating them of future pressures to smoke. The first year illustrated that resistance was highest among those with low self-esteem (Pfau et al., 1992). At the end of the second year results showed that all students in the group instilled more resistance to smoking than they did previously (Pfau & Van Bockern 1994). Importantly, the study and its follow-up demonstrate the long-lasting effects of inoculation treatments.
Compton and Pfau (2004) extended inoculation theory into the realm of credit card marketing targeting college students. They wondered if inoculation could help protect college students against dangerous levels of credit card debt and/or help convince them to increase their efforts to pay down any existing debt. The results were encouraging. Inoculation seemed to protect students' healthy attitudes about debt and some of their behavioral intentions. Also, Compton and Pfau found some evidence that those who were inoculated were more likely to talk to their friends and family about issues of credit card debt.
Inoculation is a theory developed to strengthen existing attitudes and beliefs and build resistance to future counterarguments. For inoculation to be successful it is critical that a threat (motivation for resistance) is imposed upon these existing ideas and refutational preemption (ability to cognitively build defenses to potential counterarguments) takes place after the inoculation. The argument that is presented through inoculation must be strong enough to initiate motivation to maintain current attitudes and beliefs, but weak enough that the receiver will refute the counterargument. Inoculation has been proven successful through many different trials and research. This article attempts to highlight all vital parts of the theory, however, there is a large amount of quality research on the theory that was not mentioned. Also, while numerous studies have tested the theory, there continues to be a need for improvement and new hypotheses.
See also 
- Compton, J. A., & Pfau, M. (2004). Use of inoculation to foster resistance to credit card marketing targeting college students. Journal of Applied Communication Research, 32, 343-364.
- Compton, J. A., & Pfau, M. (2005). Inoculation theory of resistance to influence at maturity: Recent progress in theory development and application and suggestions for future research. Communication Yearbook 29, 97-145.
- Gass, R., & Seiter, J. (2003). Perspectives on Persuasion, Social Influence, and Compliance Gaining. Boston, MA: Allyn & Bacon.
- Lumsdaine, A. A., & Janis, I. L. (1953). Resistance to "counterpropaganda" produced by one-sided and two-sided "propaganda" presentations. Public Opinion Quarterly, 17, 311-318.
- McGuire, W. J. (1961a). The effectiveness of supportive and refutational defenses in immunizing defenses. Sociometry, 24, 184-197.
- McGuire, W. J. (1970, February). A vaccine for brainwash. Psychology Today, 3, 36-39, 63-64.
- McGuire, W. J., & Papageorgis, D. (1961). The relative efficacy of various types of prior belief-defense in producing immunity against persuasion. Public Opinion Quarterly, 26, 24-34.
- Pfau, M. (1997). The inoculation model of resistance to influence. In F.J. Boster & G. Barnett (Eds.), Progress in communication sciences (Vol. 13, pp. 133–171). Norwood, NJ:Ablex.
- Pfau, M., Kenski, H. C., Nitz, M., & Sorenson, J. (1990). Efficacy of inoculation strategies in promoting resistance to political attack messages: Application to direct mail. Communication Monographs, 57, 1-12.
- Pfau, M., Tusing, K. J., Koerner, A. F., Lee, W., Godbold, L. C., Penaloza, L. J., Yang, V. S., & Hong, Y. (1997a). Enriching the inoculation construct: The role of critical components in the process of resistance. Human Communication Research, 24, 187-215.
- Pfau, M. & Van Bockern, S. (1994). The persistence of inoculation in conferring resistance to smoking initiation among adolescents: The second year. Human Communication Research, 20, 413-430.
- Pfau, M., Van Bockern, S., & Kang, J.G. (1992). Use of inoculation to promote resistance to smoking initiation among adolescents. Communication Monographs, 59, 213-230.