Vaccine scares, which lead portions of the population to forgo vaccination, could become more common as more diseases are eradicated, researchers say.


That's because, as cases of a disease decrease, people become complacent about their risk, and the threat of vaccines — whether imagined or real — seems greater than the threat of disease, said Chris Bauch, a professor in the department of mathematics and statistics at the University of Guelph in Ontario.


"High vaccine coverage is inherently unstable," Bauch said. Unless vaccination is mandatory, "I think we'll see more of this problem in the future," he said, referring to vaccine scares.


Vaccine scares are a problem because, when vaccination rates drop, preventable diseases such as measles and whooping cough can make a comeback. However, there may be ways to soften the blow of these scares.


Bauch and colleagues have constructed a mathematical model to predict the course of vaccine scares, including when they will peak and how long they will last.


Such models could one day be used to test the effectiveness of public health campaigns that aim to increase vaccination rates, Bauch said. For instance, the model could determine what type of campaign will work best: one that emphasizes the safety of the vaccine, or the danger of the disease, Bauch said.


To test their model, Bauch and colleagues used real data from two vaccine scares in the United Kingdom — the whooping cough vaccine scare in the 1970s and the measles-mumps-rubella (MMR) vaccine scare in the 1990s.

The researchers found their model could explain the rise and fall of vaccine coverage, and could also predict the vaccine scare outcomes.


One factor crucial to the success of the model was the inclusion of social learning, meaning the way in which people learn vaccinating behaviors from one another, Bauch said.


The model explained why it took four to five years after the start of the vaccine scares for vaccine coverage to reach its minimum. If threat of disease and the perceived threat of vaccines were the only factors that played into vaccination rates, one would expect vaccine coverage to dip significantly after people heard news reports questioning the safety of the vaccines.


However, because social learning plays a role, this was not the case. Declines in vaccine coverage take time because non-vaccinating behaviors have to "spread" from parent to parent, Bauch said.


The study will be published in the April issue of the journal PLoS Computational Biology.


Pass it on:  Vaccine scares could become more common, but mathematical models that can predict outcomes of such scares may be able to help mitigate them.


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