Even asymptomatic people carry the coronavirus in high amounts

Of all the coronavirus’s qualities, perhaps the most surprising has been that seemingly healthy people can spread it to others. This trait has made the virus difficult to contain and continues to challenge efforts to identify and isolate infected people.

Most of the evidence for asymptomatic spread has been based on observation (a person without symptoms nevertheless sickened others) or elimination (people became ill but could not be connected to anyone with symptoms).

A new study in South Korea, published Thursday in JAMA Internal Medicine, offers more definitive proof that people without symptoms carry just as much virus in their nose, throat and lungs as those with symptoms, and for almost as long.

“It’s important data, that’s for sure,” said Benjamin Cowling, an epidemiologist at the University of Hong Kong who was not involved in the work. “And it does confirm what we’ve suspected for a long time — that asymptomatic cases can transmit infection.”

Discussions about asymptomatic spread have been dogged by confusion about people who are “pre-symptomatic” — meaning they eventually become visibly ill — versus the truly asymptomatic, who appear healthy throughout the course of their infection.

The new study is among the first to clearly distinguish between these two groups.

The new study measured the virus’s genetic material in the patients; the researchers did not follow the chain of transmission or grow live virus, which might have more directly confirmed active infections.

Still, experts said the results strongly suggest that asymptomatic people are unwitting broadcasters of the virus.

“They don’t look any different from the symptomatic population” in terms of how much virus they carry, said Marta Gaglia, a virologist at Tufts University in Massachusetts who was not involved in the work.

Because asymptomatic people do not cough or sneeze, Cowling said, it is possible that they are less efficient at expelling the virus than those who are clearly unwell.

On the other hand, Gaglia offered, people who feel ill tend to take to the bed or couch, whereas the infected but unaware may carry on with their business, sickening others along the way.

The South Korean team analysed samples taken between March 6 and March 26 from 193 symptomatic and 110 asymptomatic people isolated at a community treatment centre in Cheonan. Of the initially asymptomatic patients, 89 — roughly 30% of the total — appeared healthy throughout, while 21 developed symptoms.

© 2020 New York Times News Service