Disease outbreaks partially fueled by replacement employees

When a teacher, doctor, or nurse calls in sick, someone has to take that worker’s place. The need for substitutes peaks during infectious disease outbreaks—for example, about 17% of nurses in Canada missed work at some point during the 2009 H1N1 influenza pandemic (virus pictured here). A new study by Samuel Scarpino and Laurent Hébert-Dufresne at the Santa Fe Institute in New Mexico and Antoine Allard from the University of Barcelona in Spain suggests that replacing sick workers during a disease outbreak actually accelerates the spread of the pathogen. The researchers considered a model in which some people who work in essential services were replaced shortly, though not immediately, after they got sick. By entering an environment where the disease has almost certainly spread, replacements become more susceptible to infection than an average member of the population. The total number of infected people in the model therefore increased, as did the rate of spread just before the outbreak’s peak.

Moving beyond a hypothetical disease model, the researchers looked for evidence of the replacement-worker effect in US public health data. Sure enough, Scarpino and colleagues found surges in transmission rates that occurred just before the peak of each of the 1998–2014 influenza seasons, plus the 2009 pandemic. No such pattern appeared in rates of dengue, a virus that is spread by mosquito. The researchers say their findings highlight the importance of vaccinating essential workers and the people who replace them. They also advocate for liberal sick-leave policies, since the biggest threat comes from people who work just long enough to spread the disease before finally staying home and, unwittingly, throwing a new person into the fire. (S. V. Scarpino, A. Allard, L. Hébert-Dufresne, Nat. Phys., 2016, doi: 10.1038/nphys3832 . Image credit: National Institute of Allergy and Infectious Diseases.)