Why is it so difficult to increase the number of people who vaccinate? How remains the resilience of the people against vaccination strong, even if preventable diseases are returned?
A new study by Dartmouth College suggests that past vaccine problems can lead to a phenomenon called hysteresis that creates a negative history that strengthens public interest in vaccination. The findings explain why it is so difficult to increase consumption, even if evidence suggests that vaccines are safe and useful.
A hysteresis loop causes the impact of the force to be observed even after the force has been eliminated. Therefore, unemployment rates in a recovering economy may sometimes remain high. Therefore, physical objects resist the return to their original condition after being handled by external forces. According to a Dartmouth survey, the public therefore opposes vaccination campaigns for diseases such as normal flu.
"Given all the benefits of vaccination, it was difficult to understand why vaccination rates can remain stubbornly low," said Feng Fu, assistant professor of mathematics at Dartmouth College. "History is important and now we know that hysteresis is part of the answer."
Research published in the magazine Proceedings of Royal Society B, is the first study to prove that hysteresis can affect public health.
"When people question the safety or efficacy of the vaccine, it can be very difficult to force them to move from these negative associations. Hysteresis is a powerful force that is difficult to break at the social level, "said Fu, who led the research team.
Low vaccine compliance is a public health issue that can lead to the loss of the "herd immunity" and causes the spread of communicable diseases. In some parts of Europe and North America, children's illnesses, such as measles, mumps and pertussis, have returned because of insufficient vaccine coverage.
Previous studies combined models of behavior with epidemiology to understand the challenge of voluntary vaccination but could not fully explain the existence of low vaccine compliance. Dartmouth's study specifically examines how past vaccine-related problems can affect current and future vaccination decisions.
"This study shows why it's so difficult to reverse the low or declining vaccine level," said Xingru Chen, a postgraduate student at Dartmouth and the first author of the research paper. "The very force of factual, logical arguments concerning public health issues is not enough to overcome hysteresis and human behavior."
After the study, the hysteresis loop can cause issues related to the risk and efficacy of vaccines. Negative experiences or perceptions associated with vaccination affect the trend of intake over time – known to the acquaintance as the "pathway to vaccination" that gets stuck in a hysteresis loop.
Hysteresis prevents the increase in vaccine levels even after negative objections are eliminated, making the company more and more susceptible to disease outbreaks.
"When it comes to the level of vaccination, the past predicts the future. Unfortunately, this means that many people will suffer unnecessarily if we do not find a way to break the negative impact of the hysteresis loop, "Fu said.
The study relates to the case of the entire cough vaccine in England and Wales from 1978 to 1992. This 15-year range was needed to introduce a vaccine against pertussis from 30 to 91 per cent. According to the research group, such a recovery should take only one year in ideal circumstances.
The study also notes a slow increase in measles vaccination in the event of outbreaks. In some countries, such as France, measles has become an endemic disease, although an effective vaccine is available.
According to the study: "Coverage of measles-grafting has only slowed down, but it's still insufficient for more than a decade since the infamous control of vaccination with MMR and autism."
"Vaccination rates in the population can be rapidly reduced, but due to hysteresis, the recovery of the same population may take several years," said Chen.
For a common flu, studies suggest that the vaccine should have an efficacy of over 50 percent in order to achieve a high level of vaccination, which is difficult to achieve due to the speed at which the disease mutates.
By identifying the effect of hysteresis in vaccination, the research group hopes that public health officials can set up campaigns that increase the voluntary vaccination rate, in particular by promoting vaccination as an unselfish behavior that moral and social norms want.