Drivers of the Decision to Vaccinate

Electron micrograph of a Varicella (Chickenpox) Virus. Varicella or Chickenpox, is an infectious disease caused by the varicella-zoster virus (CDC/PHIL)

When a child comes down with a fever and itchy rash without having been playing in the poison ivy, parents know that they are dealing with chicken pox.  Thankfully, chicken pox is a mild disease in childhood in most cases, especially when compared to the children’s scourges of the past, such as mumps, whooping cough, and scarlet fever, that vaccines have now mostly eradicated in the Western world.

Currently, there is a vaccine against chicken pox available to protect against the rare cases in which chicken pox is very serious and even life-threatening.  However, this vaccine is not mandatory, so there are now children growing up who are not resistant to the virus that causes chicken pox because they were not vaccinated or the vaccine was not effective in their particular case.  Unfortunately, adult cases of chicken pox tend to be much more dangerous and damaging.  As such, a low rate of chicken pox vaccination among children will indeed decrease childhood incidence of chicken pox by deceasing those who can be infected, but this would also increase the proportion of adults many years down the road who are vulnerable.

At the heart of this issue is how physicians convince patients to vaccinate themselves and their families against the dangers of any given disease, particularly a newly emergent one.  Vaccination campaigns can be highly effective at quickly arresting the spread of an infectious disease before it becomes epidemic in a population, but at the same time they are easily derailed by the wrong mixture of social ingredients.  Recent research published in PLoS Computational Biology by Drs. Flávio Coelho and Claudia Codeço* [Dynamic Modeling of Vaccinating Behavior as a Function of Individual Beliefs, published 7/10/09] develops a descriptive mathematical model of how individuals in a population will behave with regard to vaccination in an emergent disease scare.  An individual will choose whether or not to vaccinate against a new disease by deciding between the risk of death from the disease itself and the very small but ever-present risk of an adverse reaction to the vaccine itself.  By and large, people rely upon the media for new and accurate information about the world, including new diseases.  As such, Drs. Coelho and Codeço found that the media’s coverage of a vaccine, or, conversely, its coverage of adverse vaccine reactions, can significantly influence the behavior its constituent population.  “Like sometimes if someone took a vaccine and were safe,” said Dr. Coelho, “ it’s not news material, but if someone took a vaccine and died or went to the hospital, that’s news.  And it’s a fact that the news, the media industry, is biased towards things that are a lot more radical.  It introduces a bias in what people are exposed to, the kind of information that people are exposed to.”

Drs. Coelho and Codeços’ primary finding was that widespread, early coverage and bad reactions to vaccines will greatly reduce the rate at which people go to get themselves immunized against the disease.  This creates a dangerous storm in which the disease has more time to work its way through a larger susceptible population, which can make it even more difficult to contain in the long run, as well as causing a lot of suffering in the meantime.

Dr. Coelho first started with the recent MMR scares in Britain and an urban yellow fever scare in 2007 in Brazil as real-life examples to draw behavior from in developing his mathematical model.  Though he found that negative press decreased the rate of vaccination in both cases, he also found that a population’s trust in their governmental health authorities had a large influence upon that population’s behavior.  In the case of an urban yellow fever epidemic in Brazil in 2007, Dr. Coelho noted, “that scare was derived pretty much because the population did not trust that the government would tell them the truth, that they would try to cover up a real epidemic in order to avoid the political impact and thus the population decided to protect themselves no matter what the official recommendations from the Ministry of Health were.  So you see that it depends on what kind of trust relationships that is built over the handling of different similar scenarios in how the population will react.”

Drs. Coelho and Codeços’ work represents a significant advancement in the study of how diseases spread in that it takes into account both news and the public’s opinion regarding vaccines varying over time as new findings and statistics become available.   As Dr. Coelho said, “If we want to model something as complex as vaccine uptake in a population, we cannot start as has classically been done that people will vaccinate just because someone told them, or that they will vaccinate just as a fixed fraction of the population in a given situation.  It’s not as simple as that.”  Traditionally, vaccine epidemiology has assumed that populations behave homogenously, but Drs. Coelho and Codeços’ work shows that this is inaccurate in that it does not allow for individual choices for whether or not to vaccinate.  By modeling the individual decision based on best available data, and allowing both to change over time, this research has allowed for a much more nuanced view of how individual behavior in a disease outbreak influences eventual vaccine coverage, giving public health officials new and useful tools in their fight to keep the public healthy.

The West has already almost eradicated mumps, measles, whooping cough, polio, and scarlet fever.  Compared to these deadly and damaging childhood terrors, chicken pox seems less significant, but the ongoing efforts to promote vaccination against it will indeed help to protect vulnerable adults against its potentially serious complications.  And just as vaccines themselves have been, that can only be good for society.

*Dr. Flávio Codeço Coelho is affiliated with the Theoretical Epidemiology Group, Instituto Gulbenkian de Ciência, in Oeiras, Portugal and Dr. Claudia Codeco is affiliated with the Scientific Computing Program, Oswaldo Cruz Foundation, Rio de Janeiro, in Rio de Janeiro, Brazil.

[The source research maintains this column's standards in that this article is freely available to the public under PLoS's open-access policies.  I encourage you to cross-reference any comments you may have with the article itself here to broaden the public feedback loop to the researchers themselves.  A transcript of my interview with Dr. Flávio Codeço will be published 8/19/09.]