The effect of opinion clustering on disease outbreaks
Marcel Salath
Sebastian Bonhoeffer
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Institute of Integrative Biology
, ETH Zurich,
8092 Zurich, Switzerland
1
Department of Biology, Stanford University
,
Stanford, CA 94305-5020, USA
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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)