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The dynamics of vaccine scares

April 17, 2012

Vaccines are one of the greatest success stories in modern medicine. Childhood mortality has decreased dramatically over the past century, largely due to a decrease in preventable diseases. Smallpox was one of the major causes of death in Europe in the 18th and 19th century, killing about 400,000 people a year. Widespread vaccination has eradicated the disease, and there have been no known cases in more than 30 years.

Polio, a disease that destroys motor neurons in humans, has for years been close to extermination. The disease has disappeared in the Western world, and there is a concerted effort by the World Health Organization to eradicate the disease globally. However, this effort has been hindered by resistance to vaccination. For instance, in Nigeria, rumors that the vaccines were a ploy by the West to sterilize the population lead to a halt in vaccination and a resurgence of the disease.

The dynamics of vaccine scares

Vaccine scares are by no means limited to developing countries. For instance, in 1974 questions were raised in Britain about the safety of the DPT vaccine which is administered to protect children against diphtheria, pertussis and tetanus (see a nice overview in Chapter 4 here). The government initially offered a confused response to a report that suggested that the vaccine caused brain damage in a small number of children. Although the vaccine was eventually proclaimed safe, coverage dropped from 77% to 33% within 3 years, and the scare spread from Britain to the US and Japan. The result was devastating: Several major pertussis outbreaks followed, hundreds of thousands were infected and dozens died of the disease. Only after these outbreaks took their toll did vaccine coverage return to its original levels. Even today, resistance to vaccination is at least in part responsible for recent outbreaks in California and Washington State.

How can we explain such opposition to vaccines? By all accounts seeing a child with whooping caught is terrible. Patients can cough so violently that they break their ribs. So, why do vaccine scares recur?

A recent mathematical model by Chris Bauch and Samit Bhattacharyya may offer some insight. The authors used a branch of mathematics called evolutionary game theory to devise a model that takes into account how the perceived risks and benefits of vaccination are shaped by and influence disease recurrence. Here is how the model works: Most of us have never seen a child suffering from whooping cough. In the absence of direct knowledge our estimate of the danger of contracting the disease is diminished.  On the other hand, the perceived dangers of vaccination can be quite frightening. Although risks are typically small, no vaccine is completely safe. During vaccine scares, the risks, some true and most invented and unsubstantiated, come into focus. Vaccine scares then spread through social learning. As a result, the dangers of vaccination are perceived to outweigh their benefits.

When this occurs, vaccine coverage decreases. Disease outbreaks do not follow immediately, as initially the majority of the population still enjoys the protection of vaccines. Since unvaccinated individuals are still few, they enjoy protection from  those around them that have been vaccinated – a phenomenon called “herd immunity“.  Therefore, the cost of the decision not to vaccinated appears to be small initially.  However, if vaccination coverage decreases sufficiently, herd immunity breaks down, and outbreaks of the disease follow. Only after these outbreaks take their toll is vaccination coverage re-established.  As memory of these outbreaks fades, we wait for the next vaccination scare. These ideas can be described mathematically and provide an excellent description of previous vaccine scares, and the following outbreaks. Even more impressively, the model can be used to predict how vaccine scares and the resulting epidemics unfold.

Unlike our great-grandparents, we do not know multiple families who have lost their youngest to childhood diseases. Paradoxically, this has diminished our appreciation of the medical advances that have made it so much more likely that our children will survive to adulthood.


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