Why is it so challenging to increase the number of people who get vaccinated? How does popular resistance to vaccination remain strong even as preventable diseases make a comeback?
A new study from Dartmouth College shows that past problems with vaccines can cause a phenomenon known as hysteresis, creating a negative history that stiffens public resolve against vaccination. The finding explains why it is so hard to increase uptake even when overwhelming evidence indicates that vaccines are safe and beneficial.
A hysteresis loop causes the impact of a force to be observed even after the force itself has been eliminated. It's why unemployment rates can sometimes remain high in a recovering economy. It's why physical objects resist returning to their original state after being acted on by an outside force. And, according to the Dartmouth research, it's why the public resists vaccination campaigns for ailments like the common flu.
"Given all the benefits of vaccination, it's been a struggle to understand why vaccination rates can remain stubbornly low," said Feng Fu, an assistant professor of mathematics at Dartmouth College. "History matters, and we now know that hysteresis is part of the answer."
The research, published in the journal Proceedings of the Royal Society B, is the first study to demonstrate that hysteresis can impact public health.From the article, "Imperfect vaccine and Hysteresis" by Xingru Chen and Feng Fu:
Notably, in the aftermath of a sharp decline in vaccination coverage triggered by concerns regarding vaccine safety and efficacy, the recovery of vaccination rate from nadir to levels needed to attain herd immunity has been remarkably slow [39,40,55–59]. For example, it took almost 15 years for the recovery in the uptake of whole-cell pertussis vaccine from rock bottom 30% in 1978 to 91% in 1992 in England and Wales . More recently, even though resurgent measles outbreaks impose huge risks for those unvaccinated, and even in some regions like France it has become an endemic disease [57,58], the coverage of measles vaccination has only gradually climbed up, but still remains insufficient, more than a decade after the infamous MMR vaccination and autism controversy [40,59]. It seems that the recovery of vaccination rate depends not just on the extent of mitigating perceived cost of vaccination and improving vaccine efficacy, but also on the past vaccination trajectory, hence possibly marring a rapid increase.