Santa Fe Institute

Study: Replacing ill workers with healthy ones accelerates some epidemics

Aug. 1, 2016 9:44 a.m.

When disease outbreaks occur, people with essential roles – healthcare workers, first responders, and teachers, for example – are typically up close and personal with infected people. As these front-line workers become infected, healthy individuals take their places.

Mathematical biologist Samuel Scarpino, who creates and analyzes epidemiological models, wondered how this exchange of critical people affects the spread of disease. The practice clearly raises the risk of infection for the replacement individuals – but the population dynamics of this increase are neglected in existing epidemiological models. 

Scarpino, while he was an Omidyar Fellow at SFI, set out to quantify that risk and understand its influence. He enlisted the help of theoretical physicist Laurent Hébert-Dufresne, a James S. McDonnell Fellow at the Institute, and Antoine Allard, Hébert-Dufresne’s longtime collaborator from the University of Barcelona. Both study complex patterns in networks.

The trio integrated this “human exchange” into network models of disease and found that replacing sick individuals with healthy ones can actually accelerate the spread of infection. Scarpino and Hébert-Dufresne tested their ideas on 17 years’ worth of data on two diseases: influenza and dengue. Their analysis, published August 1 in Nature Physics, reveals that human exchange likely accelerates outbreaks of influenza, which spreads via human contact. But it has no effect on the spread of dengue – which makes sense, as dengue spreads via mosquitoes. 

“We didn’t see a strong signal in diseases where we didn’t expect it,” says Hébert-Dufresne. 

What does this mean in terms of practical advice for workers and managers contemplating a sick day? "Of course, being infinitely prudent and staying home as soon as we get infected would be ideal," says Hebert-Dufresne. "The problem is that we cannot afford to be infinitely prudent. From there comes a tradeoff: staying home always slows down an epidemic at first, but eventually the fact that some responsibilities of sick workers are redistributed catches up to us once the epidemic is more prevalent. That's when we see an acceleration."

A sick person should stay home, the researchers advise. But if possible, their responsibilities should be redistributed among existing workers (that is, another worker should not be brought in to replace him or her, if possible). 

Scarpino, now an assistant professor at the University of Vermont, says he hopes to see this effect integrated into future epidemiological models. “Models where you start to incorporate slightly more realistic human behavior are essential if we’re going to make high-fidelity public health and clinical decisions,” he says.

Read the paper in Nature Physics (August 1, 2016)

Read the article in Pacific Standard (August 1, 2016)

Read the article in Gizmodo (August 3, 2016)

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