The effect of opinion clustering on disease outbreaks

Journal of The Royal Society Interface, Dec 2008

Many high-income countries currently experience large outbreaks of vaccine-preventable diseases such as measles despite the availability of highly effective vaccines. This phenomenon lacks an explanation in countries where vaccination rates are rising on an already high level. Here, we build on the growing evidence that belief systems, rather than access to vaccines, are the primary barrier to vaccination in high-income countries, and show how a simple opinion formation process can lead to clusters of unvaccinated individuals, leading to a dramatic increase in disease outbreak probability. In particular, the effect of clustering on outbreak probabilities is strongest when the vaccination coverage is close to the level required to provide herd immunity under the assumption of random mixing. Our results based on computer simulations suggest that the current estimates of vaccination coverage necessary to avoid outbreaks of vaccine-preventable diseases might be too low.

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The effect of opinion clustering on disease outbreaks

Marcel Salath Sebastian Bonhoeffer Subject collections 0 Institute of Integrative Biology , ETH Zurich, 8092 Zurich, Switzerland 1 Department of Biology, Stanford University , Stanford, CA 94305-5020, USA Articles on similar topics can be found in the following collections Receive free email alerts when new articles cite this article - sign up in the box at the top right-hand corner of the article or click here - Email alerting service To subscribe to J. R. Soc. Interface go to: http://rsif.royalsocietypublishing.org/subscriptions J. R. Soc. Interface (2008) 5, 15051508 doi:10.1098/rsif.2008.0271 Published online 19 August 2008 The effect of opinion clustering on disease outbreaks Marcel Salathe1,* and Sebastian Bonhoeffer2 Many high-income countries currently experience large outbreaks of vaccine-preventable diseases such as measles despite the availability of highly effective vaccines. This phenomenon lacks an explanation in countries where vaccination rates are rising on an already high level. Here, we build on the growing evidence that belief systems, rather than access to vaccines, are the primary barrier to vaccination in highincome countries, and show how a simple opinion formation process can lead to clusters of unvaccinated individuals, leading to a dramatic increase in disease outbreak probability. In particular, the effect of clustering on outbreak probabilities is strongest when the vaccination coverage is close to the level required to provide herd immunity under the assumption of random mixing. Our results based on computer simulations suggest that the current estimates of vaccination coverage necessary to avoid outbreaks of vaccine-preventable diseases might be too low. Keywords: infectious diseases; herd immunity; clustering 1. INTRODUCTION Infectious diseases such as measles pose a constant threat to the public health worldwide. Measles, a highly contagious disease caused by a virus, is a potentially dangerous infection and a leading cause of vaccinepreventable childhood mortality (Measles. Mortality reduction and regional elimination. WHO, http://www. who.int/vaccines-documents/DocsPDF01/www573. pdf ). In many parts of the world, the incidence of measles has declined dramatically since the introduction of vaccination. The wide use of a combined measles, mumps and rubella vaccine since the 1970s has led to high measles vaccination coverage in high-income countries. In the USA, for example, measles was declared eliminated in 2000, an achievement attributed to herd immunity (Anderson & May 1991) resulting from a continued high national vaccination coverage (Anderson & May 1991; Katz & Hinman 2000). Despite high vaccination coverage, however, many countries continue to experience relatively large measles outbreaks. The general explanation for an increased frequency of such outbreaks in a given time period is that vaccination uptake has declined simultaneously. In England and Wales, for example, the drop in vaccination coverage has coincided with the number of large measles outbreaks a number of years ago ( Jansen et al. 2003). However, such an explanation cannot explain the currently observed patterns: Switzerland, for example, where vaccination coverage has steadily increased since 2000, currently experiences the largest measles outbreak since the introduction of mandatory notification for the disease in 1999 (Richard et al. 2008), with more than 2800 reported cases since the beginning of the endemic in late 2006. In 2007, England and Wales reported the highest number of measles cases since surveillance began in 1995 (Heathcock & Watts 2008). Similar patterns are observed in other countries (figure 1a). An alternative explanation for an increased incidence of outbreaks is that the vaccination coverage, even though it may be rising, is getting more heterogeneous. If an unvaccinated individual is more likely to be in contact with other unvaccinated individuals than would be expected by chance, clusters of susceptible individuals will form and thus constitute a subpopulation in which the disease can spread and cause local outbreaks. Such susceptibility clusters have been observed in the USA (May & Silverman 2003; Parker et al. 2006) and are thought to play a major role in the current outbreaks in Europe (Richard et al. 2008; Schmid et al. 2008). There is growing evidence that belief systems, rather than access to vaccines, are the primary barrier to vaccination in high-income countries (May & Silverman 2003; Parker et al. 2006; Richard et al. 2008). Vaccination exemption may have many causes such as beliefs about the safety and usefulness of vaccines, religious beliefs, philosophical considerations, etc. However, even though vaccination exemptions are deliberate choices due to a personal opinion about vaccination, susceptibility clustering does not automatically follow. What is necessary for such a susceptibility cluster to form is a process that leads to clustering of individuals (...truncated)


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Marcel Salathé, Sebastian Bonhoeffer. The effect of opinion clustering on disease outbreaks, Journal of The Royal Society Interface, 2008, pp. 1505-1508, 5/29, DOI: 10.1098/rsif.2008.0271