Around 30% of patients who tested positive for the new coronavirus were asymptomatic, but they still carried just as much of the virus as symptomatic patients—and for nearly as long—according to a new study that experts say provides the first scientific evidence of asymptomatic transmission of the pathogen.

How many Covid-19 patients have no symptoms? More than you might think.

Study details

For the retrospective study, published Thursday in JAMA Internal Medicine, researchers analyzed samples from 303 patients between the ages of 22 and 36 who tested positive for the novel coronavirus at a community treatment center in Cheonan, South Korea.

Of the 303 patients, 110 were asymptomatic before they began self-isolating. However, 21 of those individuals, or 19%, turned out to be pre-symptomatic and developed symptoms of Covid-19, the disease caused by the coronavirus, between 13 and 20 days after beginning isolation.

The remaining 89 of the 110 asymptomatic patients did not develop symptoms over the course of a 20- to 26-day follow-up timeframe, according to the researchers. However, they found that the asymptomatic patients had as large of a viral load in their noses, throats, and lungs as patients who were infected with the coronavirus and developed symptoms of Covid-19.

Further, the researchers found that asymptomatic patients carried the virus nearly as long as symptomatic patients. Almost 34% of the asymptomatic patients tested negative for the novel coronavirus after 14 days, and that percentage grew to 75% after 21 days. In comparison, 29.6% of the symptomatic patients tested negative after 14 days, and just under 70% tested negative after 21 days.

Discussion

The researchers' findings suggest that asymptomatic coronavirus patients "don't look any different from the symptomatic population" in terms of the viral load they carry, Marta Gaglia, a virologist at Tufts University who was not involved in the study, said. As a result, "[t]here's no actual reason to believe a priori that they would transmit [the virus] any differently," Gaglia explained.

Benjamin Cowling, an epidemiologist at the University of Hong Kong who was not involved in the study, added that the results "confirm what we've suspected for a long time—that asymptomatic cases can transmit infection." However, Cowling said, because asymptomatic people don't cough or sneeze, they might not be as efficient at spreading the virus as symptomatic people.

On the other hand, symptomatic patients often stay home to recover from Covid-19, while asymptomatic patients who could be unaware they're carrying the virus may continue with their everyday routines, potentially infecting others, Gaglia said.

Jason Kindrachuk, a virologist at the University of Manitoba who was not involved with the study, said many viruses can spread from asymptomatic people, but usually in very small amounts, which doesn't appear to be the case for the novel coronavirus. He added that it's still not clear whether the new coronavirus spreads at large levels from asymptomatic people because of the scale of the pandemic, or because the virus is unique in that respect.

"There are all these small nuances about this virus that are coming to light each day," he said (Mandavilli, New York Times, 8/6; Modern Healthcare, 8/6).