Hysteresis: The Phenomenon Behind the Anti-vax Movement

Despite overwhelming scientific evidence that vaccines are a safe and effective tool for the prevention of childhood diseases, a significant minority of the U.S. population remains skeptical of the practice, as evidenced by the persistence of the anti-vax movement.

This has sometimes made it a difficult task to achieve the desired level of coverage required for the protective effects of "herd immunity" to kick in. Now, researchers from Dartmouth College have investigated this phenomenon, uncovering a key factor in why it may be so hard to increase the numbers of people being vaccinated.

In a study published in the journal Proceedings of the Royal Society B, Feng Fu, an assistant professor of mathematics, and colleagues showed that a phenomenon known as "hysteresis" may act as a roadblock for efforts to increase vaccination rates.

Hysteresis can be seen in many physical systems, however, it can also be applied to human society. Put simply, it refers to the persistence of a given effect even after the conditions of the initial system have been changed.

"A hysteresis loop causes the impact of a force to be observed even after the force itself has been eliminated," Fu told Newsweek. "The occurrence of hysteresis is traditionally associated with magnetic properties of materials, and also has been found in biological and socio-economical systems. It's why physical objects resist returning to their original state after being acted on by an outside force. It's why unemployment rates can sometimes remain high in a recovering economy."

Now, Dartmouth researchers say they are the first to discover that hysteresis can arise in the context of public health interventions, in addition to fields such as physics and economics.

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

According to the researchers, the hysteresis loop can be initiated because people have negative experiences or perceptions related to vaccinations—which can stem from the fact that they can sometimes produce unwanted side effects (albeit minor in the majority of cases) and can never confer full protection against a disease. These negative experiences or perceptions can cause the "vaccination trajectory," or the trend of vaccination uptake over time, to get stuck in a hysteresis loop.

"In other words, the existence of hysteresis loop makes the population sensitive to changes in factors that drive vaccination behavior such as cost and effectiveness," Fu said. "The presence of hysteresis also makes it hard to recover to previous states, as a different path is followed."

According to first-author of the study Xingru Chen, this explains why it can be so hard to improve persistently low vaccination rates in some areas. Past research into this area has failed to fully uncover all of the factors involved in low vaccine compliance.

"The sheer force of factual, logical arguments around public health issues is just not enough to overcome hysteresis and human behavior," Chen said.

The avoidance of vaccinations leading to low vaccination coverage poses a significant problem to public health, the researchers say.

"Measles, mumps, and whooping cough, which [were] thought to no longer [be] a major threat to our society, [have made] a surprising comeback in recent years," Fu said.

However, the team hopes that the latest results could have practical implications for increasing vaccine compliance, at least partially, by overcoming the hysteresis effect.

"The identification of the hysteresis loop is a powerful finding that will help public health officials better manage vaccination campaigns," Fu said. "For example, the modeling result that shows that vaccines need to reach a certain level of efficacy to increase uptake should tell officials to focus on two things: (1) develop sufficiently efficacious vaccine and, importantly, (2) make people aware of this efficacy once it has been achieved."

"Aside from the public health prevention perspective, vaccination campaign should promote vaccination as an altruistic behavior that is desired for societal benefit (vaccination not only protects oneself but also others—including family, friends, and strangers—through the notion "herd immunity,") he said.

Recent data from the Centers for Disease Prevention and Control shows that coverage of most recommended vaccines remained stable and high in 2017 for children aged 19 to 35 months in the U.S.

However, the proportion of children who received no vaccine by the age of 24 months had increased by 0.9 percent for children born in 2011 to 1.3 percent for those born in 2015.

"This increase means that there are about 100,000 children under 2 years old that are not protected against potentially serious vaccine-preventable diseases," Amanda Cohn, Senior Advisor for Vaccines at the CDC's National Center for Immunization and Respiratory Diseases, previously told Newsweek.

And another recent study, published in the journal PLOS ONE, found evidence that growing numbers of people in certain areas of the U.S. are opposed to vaccinations.

"Since 2009, the number of "philosophical-belief" vaccine non-medical exemptions (NMEs) has risen in 12 of the 18 states that currently allow this policy: Arkansas (AR), Arizona (AZ), Idaho (ID), Maine (ME), Minnesota (MN), North Dakota (ND), Ohio (OH), Oklahoma (OK), Oregon (OR), Pennsylvania (PA), Texas (TX), and Utah (UT)," the PLOS ONE study authors wrote in their paper.

Why can it be so hard to improve low vaccination rates? iStock