Researchers in South Korea found that roughly 30 percent of those infected never develop symptoms yet probably spread the virus.
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.
“There’s been this big question pretty much since January, since data started coming out of China, about people that were asymptomatic or pre-symptomatic,” said Jason Kindrachuk, a virologist at the University of Manitoba who was not involved in the work. “What we haven’t really had any clue of yet is what role people who are asymptomatic play in transmission of disease.”
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. “There’s no actual reason to believe a priori that they would transmit any differently.”
Dr. Cowling was more circumspect. Because asymptomatic people do not cough or sneeze, he said, it is possible that they are less efficient at expelling the virus than those who are clearly unwell.
On the other hand, Dr. 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 analyzed samples taken between March 6 and March 26 from 193 symptomatic and 110 asymptomatic people isolated at a community treatment center in Cheonan. Of the initially asymptomatic patients, 89 — roughly 30 percent of the total — appeared healthy throughout, while 21 developed symptoms.
The participants were mostly young, with a median age of just 25. (A study last week found that children, who are mostly mildly infected, also harbor at least as much virus as adults do.)
“The real strength of the study is they have a very large number of patients and they have very good follow-up,” Dr. Gaglia said. “When they talk about asymptomatic patients, they really, really know that these were true asymptomatics.”
The study’s estimate that 30 percent of infected people never develop symptoms is in line with findings from other studies. In a television interview on Wednesday, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, tendered 40 percent as the figure.
“The good news about Covid-19 is that about 40 percent of the population have no symptoms when they get infected,” Dr. Fauci said. But “even though you are likely not going to get symptoms, you are propagating the outbreak, which means that you’re going to infect someone, who will infect someone, who then will have a serious consequence.”
The participants in the new study were all isolated when they tested positive for the virus and did not have the opportunity to infect others. Doctors and nurses tracked their temperatures and other symptoms, and tested their sputum — which indicates virus present in the lungs — as well as their noses and throats.
“Both groups had similar amounts of virus pretty much throughout the entire course of infection,” Dr. Kindrachuk said. Asymptomatic people became virus-free a little sooner: around Day 17, compared with Day 19 or 20 for those with symptoms.
Both estimates are much longer than the period of isolation required in most countries, Dr. Gaglia noted. The Centers for Disease Control and Prevention recently decreased the recommended isolation for infected people without symptoms to 10 days from 14.
Several studies have suggested that infected people shed — or pass into the environment — live coronavirus for only about a week, even though the tests may pick up viral fragments in their bodies for much longer.
Dr. Cowling also noted that the study was retrospective, meaning the researchers looked at samples collected from people who had tested positive earlier, instead of following a group of people over time, identifying everyone who became infected as well as their contacts, and assessing their symptoms and virus levels.
“It would still be valuable to design a study like that,” he said. Still, he conceded that comparing people with symptoms and without was challenging because infected people are found in varying ways.
Most testing plans focus on people who need medical care, and rarely whole groups regardless of symptoms — especially in places like the United States, where tests are often scarce to begin with.
A lack of testing can also influence how much asymptomatic people contribute to the size of an outbreak.
With enough testing, everyone found to be infected could be separated from others. But if the testing is barely enough to catch the visibly ill, then asymptomatic people — particularly the young and social — may fan out into society and keep the virus circulating at high levels.
Many other viruses can be spread by people without symptoms, usually at negligible levels, Dr. Kindrachuk said.
It’s still unclear whether the new coronavirus is unusual in this respect, able to spread widely from asymptomatic people, he added, or whether it just seems so prolific because of the scale of the pandemic. Studies addressing those questions are underway.
“There are all these small nuances about this virus that are coming to light each day,” Dr. Kindrachuk said.