When despite highly effective vaccine outbreaks such as preventable vaccine outbreaks such as measles, it is easy to conclude that unvaccinated parents are misled, selfish, or lost their misleading prey.

As professors with expertise in vaccine policy and health economics, we believe that the decision to not get vaccinated is more than just false information or hesitation. In our opinion, it involves game theory, a mathematical framework that helps explain how reasonable people make choices that collectively lead to the consequences that harm them.

Game theory shows that vaccine hesitation is not a moral failure, but is just a predictable outcome of a system in which individual and collective incentives cannot be properly aligned.

Game theory meets vaccines

Game theory studies how people’s results depend on how others make decisions when they choose. In his study of the subject, Nobel Prize-winning mathematician John Nash portrays “beautiful thoughts that in many cases, rational choices alone do not automatically create the best results for all.

Vaccination decisions perfectly illustrate this principle. For example, when parents decide whether to vaccinate their children with measles, they weigh the small risks of side effects of the vaccine with the risks posed by the disease. But it is a crucial insight: The risk of illness depends on the decisions of other parents. If almost everyone is vaccinated, herd immunity (essentially enough people) will stop the spread of the disease. However, once the herd immunity is achieved, individual parents may decide not to get vaccinated is a less risky option for their children.

In other words, public health goals may not be achieved due to the fundamental tension between individual choice and collective welfare.

This makes vaccine decisions fundamentally different from most other health decisions. When you decide whether to take high blood pressure medications, your results are only dependent on your choice. But with the vaccine, everyone is connected.

This interconnection has performed well in Texas, with the largest measles outbreak in the United States in a decade. As vaccination rates drop in some communities, the disease (once announced in the United States) returns. In just five years, the vaccination rate in a county has dropped from 96% to 81%. Considering that about 95% of the community must be vaccinated to achieve cluster vaccines, the decline creates perfect conditions for the current outbreak.

This is no coincidence; it is a game theory for live playback. When vaccination rates are high, it seems reasonable to have a high vaccination rate for each family, but when enough families make this choice, collective protection collapses.

Free rider problem

This dynamic creates what economists call the freerider problem. When vaccination rates are high, a person may benefit from herd immunity without receiving minimal vaccine risks. Game theory predicts something surprising: Even with the hypothetical perfect vaccine - no perfect efficacy, zero side effects - voluntary vaccination programs will never reach 100% coverage. Once the coverage is high enough, some rational people will always choose to be free riders, benefiting from the herd immunity provided by others.

And, as interest rates fall – in the past five years, disease models have predicted what we have seen sharply: the return of outbreaks.

Game theory reveals another pattern: for highly contagious diseases, vaccination rates tend to drop rapidly after safety issues, and recovery occurs much slower. This is also a mathematical property of the system, since attenuation and recovery have different excitation structures. When safety issues arise, many parents are concerned and stopping vaccination at the same time, resulting in a rapid decline in vaccination rates.

But recovery is slower because it requires both rebuilding trust and overcoming the freerider problem - every parent is waiting for others to get vaccinated first. Small changes in perception can lead to huge changes in behavior. Media reports, social networks and health news have influenced these perceptions and may shift the community to or away from these critical thresholds.

Mathematics can also predict how people’s decisions about vaccination gather. Local norms develop as parents observe other people’s choices – so the more parents skip vaccines in their community, the more others may be more suitable.

Game theorists call the low vaccine intake a pocket to produce susceptibility clusters. These clusters allow disease to persist even if the overall vaccination rate seems to be sufficient. Statewide 95% or national average may mean even vaccine coverage, which will prevent outbreaks. Additionally, this could mean that some areas have close to 100% coverage, while areas with lower dangers can cause local outbreaks.

Not a moral failure

All of this means that the sharp drop in vaccination rates is predicted by game theory, and therefore, it is more a reflection of systemic vulnerability than individual moral failures. More importantly, accusing parents of making selfish choices can also backfire, making them more defensive and less likely to reconsider their own views.

A more helpful approach is to acknowledge and work with tensions between individual and collective interests rather than being associated with psychological calculations to inform people how to make decisions in interconnected systems.

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Research shows that communities experiencing outbreaks respond differently to messaging, which uses vaccination as a community problem rather than messaging that suggests moral failure. In a 2021 study of communities with reduced vaccination rates, acknowledging the approach that parents really worry about while highlighting the need for community protection makes parents more likely to consider considering vaccination, while an approach that emphasizes personal responsibility or implying selfishness actually reduces their willingness to consider this.

This confirms the predictions of game theory: when people think their decisions are morally attacked, they are often more ingrained than more open to change.

Better communication strategies

Learn how people weigh vaccine risks and welfare points to better communicate. For example, clearly communicating the risk can help: 500 deaths from measles far outweigh the rare severe side effects of the vaccine. This may sound obvious, but public discussion is often missing. Furthermore, different communities need different approaches – high vaccination zones need help staying on track, while low vaccination zones need to rebuild trust.

Consistency is very important. Research shows that when health experts provide conflicting information or change information, people become more suspicious and decide to postpone the vaccine. Dramatic intimidation tactics about disease can backfire by pushing people to extreme positions.

Vaccination decisions can be seen within the community through community discussions and school-level reports, which can help build positive social norms. When parents understand that vaccination protects vulnerable community members, such as babies who are too young for vaccines or patients with medical conditions, it helps bridge the gap between individual and collective interests.

Healthcare providers remain the most trusted source of vaccine information. Providers can address their parents’ concerns more effectively when they understand game theory dynamics, because for most people, hesitation comes from weighing risks rather than completely opposing vaccines.