The Inaccessible: For some, vaccinating is more difficult than a short drive to the doctor’s office and a quick prick on the arm. They may live in places with a shortage of health-care resources, and have to travel a long way for an appointment. They may not have insurance. There may be a language barrier between the patient and the doctor. Often, when vaccination is inconvenient, “attitudes are not strongly against or in favor of vaccination ... which means that vaccination is not important enough to actively overcome barriers,” the researchers write. Inaccessible Ian lives in a rural area, and has to drive an hour and a half to see his doctor, who he pays out of pocket, because he doesn’t have insurance. He sees no reason to make the trip when his son’s perfectly healthy right now.

The Calculating: “They engage in an extensive information search for pros and cons of vaccination,” the article reads. “If the risk of infection is perceived to be lower than the risk of vaccination, the decision will be against vaccination … This can result in free-riding, that is, the idea that it is selfish-rational to omit vaccination as long as enough other individuals are vaccinated to keep the infection risk low.” Calculating Carl knows a lot about herd immunity. After his daughter was born, he spent hours online researching vaccines. Though he thinks vaccines are probably safe, he’s seen enough people questioning to have just a seed of doubt. And since his research tells him that as long as most people vaccinate, the whole population should be safe, he thinks, why expose his daughter to even the smallest risk if she’ll be protected anyway?

The Unconfident: These are the classic anti-vaxers, the ones who get the most attention. They don’t trust that vaccines are safe, they don’t trust the health system or the doctors telling them vaccines are safe, and/or they don’t trust the people who make the policies promoting vaccination. Whereas the calculating and the complacent have more of an ambivalent attitude toward vaccines, the unconfident feel negatively about them. Unconfident Ursula is worried. A friend of hers told her he knows someone whose kid was diagnosed with autism a few months after getting vaccinated. Though her doctor has assured her there’s no link between vaccines and autism, she doesn’t feel like he’s taking her concerns seriously enough. Her friend says he’s thinking about opting out of vaccinating his kids, and Ursula is starting to think maybe she should, too.

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“We suggest that different types of non-vaccinators have different sets of ‘active determinants’ that influence their decisions,” the researchers write. “Furthermore, we propose that interventions should be targeted to these differences.”

For example, Inaccessible Ian needs structural help to make vaccinating easier and more affordable. A campaign that explains the risks of not vaccinating, and the benefits vaccines have for society, could change Calculating Carl's calculations, and it could shock Complacent Courtney out of her complacence. Calculating Carl could also be motivated by financial incentives.