There is a reason some schools on the coast of California only have a 25 percent vaccination rate. Not 25 percent exemption, 25 percent vaccination. In California overall, exemptions rates rose by 25 percent just between 2008 and 2010 and that trend has been mirrored in states like Washington and Oregon.

The recent rash in anti-vaccination hysteria can be mathematically modeled, say scholars from the University of Guelph and the University of Waterloo.

Writing in Proceedings of the Royal Society B, the authors say their model can foresee the observed patterns of population behavior and disease spread during vaccine scares - times when anti-vaccine sentiment is strong. That means they should be able to see it even when it is just part of the beliefs of people who influence each other's social norms by moving to areas where people are more 'like' them.

"If vaccination is not mandatory and disease is rare, then a few parents will be tempted to stop vaccinating their children," said senior author Professor Chris Bauch of Waterloo's Faculty of Mathematics. "More parents adopt this behavior as social norms begin to change and it becomes increasingly acceptable to avoid some vaccines. Obviously, when enough parents are no longer vaccinating, the disease will come back." 

That happened with Whooping Cough. In 2010, California school-aged children only had a 91 percent vaccination rate, well below the 95 percent herd immunity rule of thumb. The typical exemption rate across the U.S. was 1.2 percent. Farther inland, people are more accepting of science and medicine but on the coast, the anti-vaccination rates are far higher. And that is where the Whooping Cough epidemic took hold in 2010.

It comes down to vaccine fear culture, the authors say, and that can be predicted. In most of North America, pediatric vaccination is mandatory for children enrolled in public education but the number of parents applying for exemptions to pediatric vaccination is on the rise.  States like Mississippi and Alabama look absolutely rational when it comes to medicine, while states like Washington and Oregon seem to be motivated by fear and conspiracy, even though they claim it is because they are more informed. That normative behavior can be modeled in a predictive way, the authors believe.

They hope to use this model to create an index, which may be able to help determine which populations are more susceptible to vaccine scares, with the hope of preventing them from occurring. We can do that right now; draw a circle around a Whole Foods store and look at voter registrations, the same way we can model acceptance of climate change by demographics and therefore geography. 

"Parents are not cold, clinical rationalists who base their decisions only on data. They are strongly influenced by other parents and what they read," said Bauch. "Our research suggests that health officials needs to have a really good understanding of the social context to better understand vaccine scares and why people refuse vaccines. To do that, we have to develop predictive tools that also reflect social behavior patterns, or we won't be able to accurately represent what is happening during vaccine scares. 

"If you've seen a big drop in vaccine coverage and you've seen a surge of disease because of that, you can use these models to predict how long it will take vaccine coverage to recover." 

Citation: Tamer Oraby, Vivek Thampi, and Chris T. Bauch, 'The influence of social norms on the dynamics of vaccinating behaviour for paediatric infectious diseases', Proc. R. Soc. B. 2014 281 20133172; doi:10.1098/rspb.2013.3172