Harvard Medicine

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Win Friends and Influence Influenza

Photo by Ken Brown<br/>iStockphoto.com Your friends are probably more popular than you are. Curiously, this “friendship paradox” may help predict disease outbreaks.

Noted nearly two decades ago, the friendship paradox holds that, statistically, the friends of any given individual are likely to be more popular than she is. Take a random group of people, ask each to name one friend, and on average that friend will rank higher in the social web. And those popular friends will collect gossip, trends, ideas—and, it seems, diseases—sooner than their less connected counterparts.

Monitoring those pivotal members of social networks is an ideal way to predict outbreaks, Nicholas Christakis ’88, an HMS professor of medicine and of medical sociology, and James Fowler of the University of California, San Diego, found when they applied the friendship paradox to the 2009 flu epidemic. As influenza season approached, they contacted 319 Harvard undergraduates who, in turn, named 425 Harvard friends. And lo: those friends manifested flu symptoms up to 46 days sooner than the rest of the student population. The study appeared September 15 in PLoS ONE.

“Public health officials often track epidemics by following random samples of people or monitoring people after they get sick,” says Christakis. “But by simply asking members of the random group to name friends, and then following both groups, we can predict epidemics before they strike the population at large.”

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