Asymptomatic People Carry Similar Levels of Coronavirus as Those with Symptoms in Study

People who show no signs of being infected with the coronavirus have been found to carry similar levels of the germ in their bodies than those with, in a study on South Korean patients.

Following one of the first major COVID-19 outbreaks outside China in February, South Korea started mass-testing and contact tracing. This included keeping those who had tested positive—but didn't have symptoms—in community treatment centers.

Scientists studied 303 people who had tested positive for the coronavirus between March 6 and 26. They were isolated at a community treatment center in the city of Cheonan. Samples from their upper respiratory tract (inside the nose) and lower respiratory tract (in the form of spit) were taken. The patients were tested eight, nine, 15 and 16 days after they were isolated. On days 10, 17, 18 and 19, doctors tested samples from the patients' upper or lower respiratory tract at their discretion.

Of the total patients, 193 were symptomatic when they were isolated, as opposed to 110 who were asymptomatic. The most common symptoms included a cough, nasal congestion, and producing more spit than normal. Of the 110 patients without symptoms, 21, or just under a fifth, developed symptoms while they were isolated. On average, it took 15 days for patients who did not show symptoms to do so.

By comparing the levels of the virus' genetic material in the samples, the team found the viral loads were similar whether patients were symptomatic or asymptomatic.

However, writing in the journal JAMA Internal Medicine, co-author Dr. Seungjae Lee, of the Department of Internal Medicine at South Korea's Soonchunhyang University Seoul Hospital, and colleagues said they were unable to determine if this meant these individuals were able to pass on the virus as easily as those with symptoms.

Also, finding the genetic material of the virus in a person does not necessarily mean they can pass it on, and more research is needed to explore this question, they said. But the high viral load they found "raises a distinct possibility of a risk for transmission," they wrote.

The study implies that only testing people who have symptoms could result in "substantial underreporting of infected patients," according to the team.

The study was limited, the team said, for a number of reasons, including that the participants were generally young and otherwise healthy, and so the results may not relate to the rest of the population.

Ian Jones, professor of virology at the U.K.'s University of Reading who did not work on the paper, told Newsweek: "All individuals were isolated and transmission was not tested so it is impossible to know if, generally, asymptomatics would transmit less virus to others, for example because they do not cough, or more as a result of freedom of movement.

"However, the study serves to reinforce the point that control of COVID-19 in the community requires regular widespread testing and that anyone positive should self-isolate whether they are symptomatic or not."

Daniel Davis, professor of immunology at the University of Manchester, U.K., who was also not involved in the study, described the study as "interesting and important"

Davis, the author of The Beautiful Cure, a book about the human immune system, said the findings that asymptomatic and symptomatic patients had similar viral levels are consistent with the idea that people showing no symptoms might still be able to spread the virus.

"This means that a 'track and trace' system might feasibly miss a significant number of people spreading the virus when they don't show any symptoms. However, this study didn't look at transmission of the virus directly. It's hard to infer rates of transmission from the types of measurements done here."

Jeremy Rossman, honorary senior lecturer in virology at the University of Kent who was also not involved in the study, told Newsweek: "We have known that most virus transmission occurs early during the infection, with much transmission in the pre-symptomatic period. However, the contribution of asymptomatic transmission has been less clear.

"Here we see that asymptomatic and symptomatic patients have comparable amounts of virus in their respiratory tracts." Rossman said it is a plausible assumption that the comparable levels of virus equate to comparable transmission, and could affect public health policy.

"We need to be testing widely and to be performing robust contact tracing in order to identify all contacts and new potential cases," he said.

This article has been updated with comment from Jeremy Rossman.

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People wear protective face masks on August 3, 2020 in New York City. Scientists have investigated whether people who are asymptomatic carry different levels of the virus. Noam Galai/Getty Images