This Is Who's Transmitting Over 50 Percent of COVID Cases, Study Finds

As the coronavirus pandemic surges across the U.S., these findings could help avoid "a vast outbreak."

Officials are struggling to keep mounting coronavirus cases in check as areas of the U.S. continue to see back-to-back record-breaking days of new infections. Fortunately, medical experts are becoming more confident that the spread of the disease can be slowed by practicing social distancing, wearing a face mask, and increased testing to avoid new outbreaks from cropping up. But the latter may be especially important, based on new research. According to a recent study out of Yale University's Center for Infectious Disease Modeling and Analysis, infected patients with no symptoms are responsible for transmitting more than half of COVID-19 cases.

"Our results indicate that silent disease transmission during the pre-symptomatic and asymptomatic stages are responsible for more than 50 percent of the overall attack rate in COVID-19 outbreaks," the authors write in the study, which was recently published in the journal Proceedings of the National Academy of Sciences. Pre-symptomatic patients are those without symptoms who eventually show signs of the virus, while asymptomatic patients never develop noticeable symptoms of the disease.


The study based its findings on a model that was created using existing research in an attempt to determine the best methods of containing the novel coronavirus. Because the majority of cases "may be attributable to silent transmission from a combination of the pre-symptomatic stage and asymptomatic infections," the authors write, "even if all symptomatic cases are isolated, a vast outbreak may nonetheless unfold."

The findings add to mounting evidence that "silent spreaders"—or patients who are unaware that they are infected and contagious—are a major contributor to community outbreaks of COVID-19. Since patients without symptoms are unlikely to refrain from social interactions, this puts them at a greater likelihood to be transmitting COVID-19, the study posits.

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As a result, the authors suggest using a combination of techniques, including increased testing and contact tracing, to control the coronavirus. "Our findings highlight the urgent need to scale up testing of suspected cases without symptoms," they write. "Delays in contact tracing increase the risk of onward transmission, especially since those without symptoms are generally unaware of their infection risk to others, and therefore are less likely to curtail social interactions."

On Twitter, the study's lead author, Alison Galvani, PhD, professor of epidemiology at Yale University, said that the results prove that "symptom-based control, such as temperature checks, is not sufficient" to stop coronavirus from spreading. And for more on the guidelines to follow, check out Here's How Long You'll Have to Wear a Face Mask, Experts Say.

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Zachary Mack
Zach is a freelance writer specializing in beer, wine, food, spirits, and travel. He is based in Manhattan. Read more
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