Explaining public resistance to vaccination

“Once people question the safety or effectiveness of a vaccine, it can be very difficult to get them to move beyond those negative associations. Hysteresis is a powerful force that is difficult to break at a societal level,” said Fu, who led the research team.

Low vaccine compliance is a public health issue that can cause the loss of “herd immunity” and lead to the spread of infectious diseases. In parts of Europe and North America, childhood diseases like measles, mumps and pertussis have returned as a result of insufficient vaccination coverage.

“We have great evidence that vaccines work and prevent illnesses that can create a lot of morbidity and even death in children,” says Ellen Meara of the Dartmouth Institute. “And we have really no scientific evidence suggesting that they are unsafe in the way that resistance toward vaccinations suggests. It does feel like a public health crisis.”

Previous studies have combined behavior models with epidemiology to understand the challenge of voluntary vaccination, but have been unable to completely explain the persistence of low vaccine compliance. The Dartmouth research specifically studies how past problems associated with vaccinations can impact present and future vaccination decisions.

“This study shows why it is so hard to reverse low or declining vaccine levels,” said Xingru Chen, a graduate student at Dartmouth and the first author of the research paper. “The sheer force of factual, logical arguments around public health issues is just not enough to overcome hysteresis and human behavior.”

According to the research, the hysteresis loop can be caused by questions related to the risk and effectiveness of vaccines. Negative experiences or perceptions related to vaccination impact the trend of uptake over time — known to the researchers as a “vaccination trajectory” that gets stuck in the hysteresis loop.

Hysteresis prevents an increase in vaccination levels even after the negative objections have been cleared, making society increasingly vulnerable to disease outbreaks.

“When it comes to vaccination levels, the past predicts the future. Unfortunately, this means that a lot of people are going to needlessly suffer unless we find a way to break the negative impact of the hysteresis loop,” said Fu.

Chen and Fu expressed frustration in the confusion between disease and the side effects of vaccinations, which perpetuates fear of vaccines. Equally problematic is the free-rider problem, in which people refuse vaccines because they rely on the majority of their community to get treatment.

“Even if side effects are minor, it makes people exaggerate the risks of the vaccine,” Fu said. “Because of the massive preemptive vaccination of childhood diseases, people don’t see the diseases that often. So people replace the fear of disease with the fear of the vaccine. If you want to boost the vaccination levels, it [will happen] slowly because people get stuck in a hysteresis loop.”

>To amplify the problem, public misconceptions surrounding vaccines are amplified due to what Fu calls an “echo chamber” where people surround themselves by others who will only reconfirm their biases.

The study refers to the example of whole-cell pertussis vaccine in England and Wales in the period from 1978 through 1992. It took that 15-year span for uptake of the “whooping cough” vaccine to recover from 30 percent to 91 percent. According to the research team, such a recovery should only take about a year under ideal circumstances.

The research also notes the slow increase in measles vaccination in the face of resurgent outbreaks. In some countries, like France, measles has become an endemic disease despite the availability of an effective vaccine.

According to the study: “The coverage of measles vaccination has only gradually climbed up, but still remains insufficient, for more than a decade following the infamous MMR vaccination and autism controversy.”

“Vaccination levels in a population can drop quickly, but, because of hysteresis, the recovery in that same population can take many years,” said Chen.

For the common flu, the study suggests that a vaccine would have to have an effectiveness of above 50 percent in order to achieve high levels of vaccination, a difficult level to reach because of the speed with which the illness mutates.

By identifying the hysteresis effect in vaccination, the research team hopes that public health officials can design campaigns that increase voluntary vaccination rates, particularly by promoting vaccination as an altruistic behavior that is desired by moral and social norms.

Tackling vaccination hesitancy
Despite the scope and magnitude of this trend, Chen and Fu were determined to use their expertise in mathematical modeling to tackle a global problem.

“We are doing research using mathematics to explain some social phenomenon,” Chen said. “But I think it’s beyond our ability to persuade the audience and normal people to do something. We also received hate emails after we published the paper.”

Fu said that mothers were mainly responsible for the hate mail.

“I can understand, because MMR side effects like fever and rashes look terrible, but the measles and mumps and whooping cough are even worse,” he said. “Instances of the diseases are not huge, so people don’t confront these diseases very often. Now they don’t fear diseases, but they fear vaccines and their side effects. Vaccines became the victim of their own success.”

To increase vaccination levels, the researches pointed to decreasing herd immunity, which means that people forgo vaccinations and rely on the fact that a great proportion of their community will get vaccines instead.

“One way to overcome the hysteresis effect is just to increase altruistic behavior,” Fu said. “The vaccine compliance problem is essentially a free-riding problem.”

Meara also noted that introducing evidence and information to those who are resistant toward vaccines can be much more effective than shaming those individuals.

“One of the things I found really interesting is a local provider who is making an effort to not shame clients who have a fear of vaccines, but instead to share information and evidence,” Meara said. “Instead of pushing it, what she does is she reaches out individually to parents to let them know [about disease outbreaks.] Apparently, that kind of soft approach can be really effective in bringing someone around.”

Though it may be difficult to boost altruistic behavior and break the hysteresis loop, Chen and Fu’s research has highlighted a global health issue and proved the necessity of mathematical modeling in real-world problem solving.

“It’s important for future scientists to learn not just mathematics but [an] interdisciplinary approach to real world problems,” Fu said. “Models can be very profound and have [a] huge impact [on] reshaping our thinking of global problems.”

— Read more in Xingru Chen and Feng Fu, “Imperfect vaccine and hysteresis,” Proceedings of the Royal Society B: Biological Sciences (9 January 2019) (DOI: 10.1098/rspb.2018.2406)