Hysteresis loop explains resistance to vaccination, researchers say

Jan. 9 (UPI) -- Researchers say that a hysteresis loop explains why vaccination rates for some diseases continue fall, even after health officials declare vaccines safe, a new study says.

Past concerns or troubles with vaccines that researchers have since explained or fixed continue to keep many people away from them, researchers at Dartmouth College suggest in a study published Wednesday in Proceedings of the Royal Society B.

A hysteresis loop occurs when the negative impact of a particular force continues to linger in the mind of a person even when that negative impact current threatens a current situation. Unemployment rates remaining high during great economic times after a recession is one example of a hysteresis loop.

"Given all the benefits of vaccination, it's been a struggle to understand why vaccination rates can remain stubbornly low," Feng Fu, an assistant professor of mathematics at Dartmouth College and study author, said in a news release. "History matters, and we now know that hysteresis is part of the answer."

Low vaccine compliance has created a public health problem that can allow an infectious disease to spread because it damages "herd immunity," which is the immunity to particular diseases built up by group immunity and often increased through mass vaccination of populations.

Areas of Europe and North America are currently facing a resurgence of diseases such as measles, mumps and pertussis, childhood conditions that have flared due to lack of vaccination coverage.

The study says that a hysteresis loop can arise from questions about the risk of the vaccines.

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.

A Centers for Disease Control and Prevention survey from mid-November found that flu vaccination rates remained low, even after a year that saw a historic number of deaths and hospitalizations due to the flu.

Health officials in New York have linked its worst measles outbreak ever to low vaccination numbers in the state. They attribute the decrease to the promotion of anti-vaccine messages and poor school vaccination enforcement requirements.

Much of the fear of vaccinations has developed from a belief among parents that ingredients in certain vaccines can cause autism. A 2013 CDC study, however, attempted to dispel that idea.

In 2017, measles claimed more than 100,000 lives and Europe continues to deal with outbreaks of the disease. Much of the crisis is due to low vaccination coverage for measles.

"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," Fu said.