Researchers study hysteresis in vaccination decisions

by Cassandra Thomas | 1/22/19 3:10am

Vaccines were first introduced two centuries ago as a disease prevention mechanism. Since then, medical professionals have used them routinely for their consistently safe and beneficial effects. However, recent research by mathematics professor Feng Fu and graduate student Xingru Chen has demonstrated that decreasing vaccination rates in developed countries are worsened by the hysteresis effect.

Hysteresis is a phenomenon in which a system is continually affected by its past, even if previous forces have been long removed, according to both Chen and Fu. In terms of vaccination, this means that once people question the efficacy of vaccines, it can take decades for vaccination rates to recover to their original levels. Chen and Fu’s study revealed that this pattern exists globally.

Chen and Fu model the vaccination dynamics as a two-stage game. In the first stage, people make decisions about whether or not to get vaccinated, which will later determine their risk of infection. In the second stage, health outcomes determine individuals’ payoffs. 

Fu stated that their research was prompted by alarming trends that revealed a comeback of childhood diseases like measles and mumps. Typically, these are treated with the measles, mumps and rubella vaccines. But vaccination rates have not been able to recover since a false claim was made that suggested a link between autism and the MMR vaccine.

Health economics professor at The Dartmouth Institute Ellen Meara said that as an individual who cares about public health, she has been watching the diving vaccination rates with concern and interest.

“We have great evidence that vaccines work and prevent illnesses that can create a lot of morbidity and even death in children,” Meara said. “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.”

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.

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.”