The seemingly irrational resistance to vaccines in parts of the developed world has baffled policy makers for years, even as the diseases the vaccines are designed to prevent make a comeback.  A new study from researchers at Dartmouth College explores some of the reasons why this resistance persists, even in the face of mounting evidence.

It creates something known as hysteresis, which causes the impact of a force to be observed, even when the force itself has long since been eliminated.

“Given all the benefits of vaccination, it’s been a struggle to understand why vaccination rates can remain stubbornly low,” the authors explain. “History matters, and we now know that hysteresis is part of the answer.”

Shifting opinions

The resistance to vaccination in parts of Europe and North America has seen an unwelcome return of diseases such as measles, mumps and pertussis.  Previous studies have attempted to understand things but have struggled to satisfactorily explain why resistance persists even as evidence mounts up.

The logical argument is largely failing to convert people, so better understanding why people stubbornly persist has significant implications.

The researchers reveal that the hysteresis loop is triggered by questions related to the relative risk and effectiveness of the vaccine.  Negative actual experiences or perceptions have a clear impact on adoption over time, with hysteresis contributing to high levels of resistance even when negative objections have been cleared.

“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,” the researchers explain.

Monitoring adoption

The researchers tracked the adoption of the whole-cell pertussis vaccine in England and Wales between 1978 and 1992.  At the start of this period the vaccine had a 30% adoption rate, and it took until 1992 for it to recover to more acceptable levels of 91%.  In ideal circumstances, this kind of recovery would take a year rather than 15 years.

What’s more, this recovery doesn’t seem to increase in pace when outbreaks recur.  For instance, the researchers cite France, where measles has become an endemic disease again despite the ready availability of a vaccine.

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

The team hope that by identifying the hysteresis effect, it will help public health officials better design campaigns to increase the voluntary vaccination rates, with a possible avenue being to promote vaccines as an altruistic behavior.