Published by Communications and Public Affairs (519) 824-4120, Ext. 56982 or 53338

# News Release

August 23, 2004

## Perceived risk influences vaccination rates, new study finds

Why do preventable diseases such as measles and whooping cough seem to be making a highly contagious comeback? New research by a University of Guelph professor may provide one possible explanation.

In a study published today in the Proceedings of the National Academy of Sciences, mathematician Chris Bauch analyzes how the perceived risk of vaccines among the general public influences vaccination rates and disease outbreaks.

He developed a mathematical model with professor David Earn at McMaster University that predicts how much vaccination rates will drop following a large increase in perceived risk (e.g. a “vaccine scare”).

To build their model, the researchers combined game theory with epidemic modelling to show how people make decisions about vaccination. Game theory is a mathematical analysis of how individuals make decisions when the personal effect of their decision depends on the decisions reached by others in a group.

“A parent’s decision to vaccinate is indirectly influenced by the decisions of all other parents because the risk of infection depends on the proportion of the population that is vaccinated,” said Bauch.

“Those vaccination decisions are also influenced by the perception of risk, whether accurate of inflated.”

In making such decisions, parents typically consider the risk of morbidity from vaccination, the probability that their child will become infected if not vaccinated and the risk of morbidity from such an infection, he said.

Under normal circumstances, the risk of vaccination is perceived to be low. During a "vaccine scare," people are influenced by numerous factors such as media coverage and the activities of anti-vaccination groups, and the perceived risk of vaccination can rise dramatically.

For their study, Bauch and Earn looked at vaccines for the childhood diseases measles, whooping cough, chickenpox, mumps and rubella. Their choice was motivated by the drop in vaccination rates in Great Britain for the so-called “triple jab” vaccine for measles, mumps and rubella (MMR). The drop followed a 1998 report that suggested a potential link between the vaccine and autism.

“Our approach allowed us to quantify how risk perception influences vaccine uptake levels and what role is played by the epidemiological characteristics of the pathogens,” Bauch said. “It's applicable to any country that has voluntary vaccinations.”

Bauch and Earn conclude that “it will be relatively easy to induce a drop in vaccine uptake during a scare, but relatively difficult to restore levels afterwards, even if perceived vaccine risk is greatly reduced.” Vaccine scares are often followed by educational campaigns aimed at increasing public confidence, but the researchers explain mathematically how the effectiveness of such programs is constrained.

The MMR scare in Great Britain was “based essentially on a hunch, not on scientific evidence,” Bauch said. Numerous scientific studies have been published since 1998 that did not find any statistical link between the MMR vaccine and autism, “but people are still worried, so they are not vaccinating their kids.”

As well, surveys in the UK have shown that a significant proportion of parents believe the vaccine carries more risk than the diseases against which it protects. “If this trend continues, measles epidemics in Great Britain are inevitable,” said Bauch.

This latest research is another example of how game theory can help in the evaluation and development of public health policy, Bauch said. He published a report a year ago on game and voluntary vaccination for smallpox. That study also appeared in the Proceedings of the National Academy of Sciences, one of the world's most-cited multi-disciplinary scientific serials.

Contact:
Prof. Chris Bauch, Department of Mathematics
(519) 824-4120, Ext. 53079
cbauch@uoguelph.ca

For media questions, contact Communications and Public Affairs: Lori Bona Hunt, (519) 824-4120, Ext. 53338, or Rachelle Cooper, (519) 824-4120, Ext. 56982.

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