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Asymptomatic COVID-19 cases may be more common than suspected

The problem? Even without symptoms, people can still spread the virus.
Image: Venice Beach
People walk at the boardwalk in Venice Beach during the first day of the Memorial Day holiday weekend May 23, 2020, in Los Angeles.Apu Gomes / AFP - Getty Images

New estimates of the number of asymptomatic people with the coronavirus suggest that "silent" COVID-19 is much more prevalent than once thought, according to two studies published Wednesday.

The first study, published in JAMA Network Open, found that 42 percent of cases from a group of people in Wuhan, China, were asymptomatic. The second study, published in Thorax, found much higher rates of asymptomatic individuals: 81 percent of cases on a cruise to Antarctica.

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The study from Wuhan looked at 78 patients who tested positive for COVID-19, and found that 33 of the individuals had no symptoms of the illness. These patients were more likely to be women, and more likely to be younger, in their 20s, 30s and early 40s.

Meanwhile, the second study, from Australian researchers, looked at 217 people on a cruise bound for Antarctica. The ship set sail in mid-March, just after the World Health Organization declared the coronavirus a pandemic.

The first fever on board was reported eight days into the voyage. Over the following two weeks, eight people had to be evacuated from the ship because they fell ill.

All of the 217 people who remained on board were tested for COVID-19. More than half (59 percent) tested positive, but just 19 percent of those patients had symptoms. The other 81 percent were symptom-free.

"Many people still haven't grasped the notion that asymptomatic people can be so common, and they wonder why it is they have to wear the mask when they're feeling well, or why they have to keep doing this social distancing stuff," Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Nashville, said.

"Simply exhaling can send out viral particles," said Schaffner, who wasn't involved with either study.

That's why the CDC encourages everyone to wear face coverings or masks in public to help prevent the spread of the virus. The agency's estimate of the prevalence of asymptomatic cases, based on mathematical modeling, is lower, at 35 percent.

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There was one positive finding, however, from the study in China: Asymptomatic individuals may not spread the virus for as long as symptomatic patients do. The patients without symptoms shed the virus for about eight days, compared with 19 days among those who did have symptoms, the researchers, from Zhongnan Hospital of Wuhan University, found.

Still, asymptomatic COVID-19 cases remain a concern.

"This is very important because, theoretically, you can spread the infection when you're shedding the virus because it's so highly contagious," Dr. Aditya Shah, an infectious disease fellow at Mayo Clinic in Rochester, Minnesota, said.

Though COVID-19 has proven it has the ability to sicken anyone at any age, people over age 65 and those with underlying chronic health conditions, such as type 2 diabetes, appear to be most vulnerable.

"I don't know of a single person, no matter how independently-minded they are, who has any desire to give this virus to anyone else," Schaffner said. "But they have to recognize that they could."

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