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Outbreaks Hinge on Superspreaders

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A research team led by CNR experts recently found that diseases such as Severe Acute Respiratory Syndrome (SARS) and measles are prone to situations in which a few people, given the right conditions, can ignite explosive epidemics. However, the researchers say that the volatility of these “superspreading events” also means that outbreaks are likely to fizzle out relatively quickly.

“From Typhoid Mary to SARS, it has long been known that some people spread disease more than others,” says James Lloyd-Smith, a post-doctoral researcher in the Department of Environmental Science, Policy, and Management (ESPM). Lloyd-Smith is the lead author of a recent study of deadly diseases that showed that a small subset of particularly infectious people can exert a powerful influence over how outbreaks progress.

“For many diseases, we found that the proportion of infected people who do not infect anyone else is higher than previously expected, suggesting that health officials should not be lulled into complacency by an absence of flu transmission events,” notes ESPM professor Wayne Getz, the principal investigator of the study.

The characteristics of a “superspreader,” or an especially infectious individual, depend upon the disease and other factors. For example, an individual is at greater risk of being a superspreader if his or her job requires frequent, close contact with a large number of people. Health professionals, for instance, are at greater risk for both contracting and spreading certain diseases. This was illustrated all too clearly with the 2003 SARS outbreak that was traced to a Hong Kong hotel where an infected physician from China had stayed.

“For diseases like SARS, major outbreaks occur when the disease hits the jackpot by infecting a superspreader,” says Lloyd-Smith. “A superspreader could go on to infect 10, 20, or even more people if the conditions are right. In one extraordinary case, a sailor transmitted measles to about 250 other people in Greenland.”

The study quantifies what many health professionals have long suspected, and has important implications for emerging disease surveillance and control.

"During the start of an epidemic, public health agencies need to pay the same attention to how many patients are not transmitting the disease as they do to counting the cases of those that transmit widely,” says Getz. This could help officials predict and avoid superspreading events in the first place.

-Sarah Yang

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