Understanding transmission by asymptomatic patients has been cited as a key factor in controlling the spread of COVID-19 in communities. Korean public health authorities have repeatedly warned of the potential of asymptomatic patients to be behind a “silent spread” of the disease.

“Epidemiological investigations reveal approximately 40 percent of all confirmed cases so far are asymptomatic,” said Korea Disease Control and Prevention Agency’s deputy chief Kwon Jun-wook in a Tuesday briefing. The Seoul metropolitan office said Monday that among the patients confirmed last week, as many as 31.2 percent did not manifest any symptoms.

So just how common are asymptomatic patients -- and how infectious are they?

Asymptomatic patients can carry the coronavirus just as much as symptomatic patients, and for as long, according to a study published last month in the Journal of the American Medical Association.

The study looked at 303 mildly sick patients at a nonhospital treatment facility in Cheonan, South Chungcheong Province, from March 6 to 26 to find that 36.3 percent did not present with symptoms at the time of diagnosis.

While the study did not establish how infectious asymptomatic patients can be, the high viral load observed in them raises “a possibility of a risk for transmission,” said Soonchunhyang University Hospital’s infectious disease specialist Dr. Kim Tae Hyong, who supervised the study.

Another key implication of these findings is that “the current symptom-based screening may miss asymptomatic as well as other less obvious cases,” he said.

A recent study published by the Journal of Korean Medical Science suggested the contrary, however, arguing that asymptomatic infection may be much lower than previously reported.

The study, which investigated 40 COVID-19 patients isolated at a hospital in Ulsan between Feb. 22 and March 26, said only two remained without symptoms until recovery.

“Upon uninterrupted observation of the clinical course of COVID-19, symptoms developed in five of the seven patients who were initially reported to be asymptomatic,” the study said. “This means careful interviews and follow-ups are necessary to determine true asymptomatic infections.”

Multiple factors have to be taken into consideration in estimating the proportion and transmission potential of asymptomatic patients, according to infectious disease expert Dr. Choi Won-suk of Korea University Medical Center in Ansan, Gyeonggi Province.

“The proportion of asymptomatically-infected individuals can vary depending on how a survey is designed,” he said.

The distinction between asymptomatic and very mildly symptomatic cases was also rather blurred. “Symptoms can be subjective in the minimally sick patients. Some of them may never end up reporting symptoms,” he said.

Choi pointed out asymptomatic patients were less likely to socially distance as well. “People with little to no symptoms can carry on with daily routines without realizing they are sick, possibly exposing more people to the disease along the way,” he said.

By Kim Arin (arin@heraldcorp.com)