People who are infected with the coronavirus carry similar levels of the pathogen in their nose, throat and lungs whether they have symptoms or not, a new study from South Korea showed Thursday.

The paper, published in JAMA Internal Medicine, is an important biological line of evidence in support of the idea that asymptomatic carriers can spread COVID-19.

Until now, experts have relied on inferring asymptomatic spread when people contract the virus without contact with a known carrier.

A team of researchers led by Seungjae Lee at Soonchunhyang University College of Medicine analyzed swabs taken between March 6 and March 26 from 303 people isolated at a center in Cheonan, following an outbreak among a religious group in another city.

The group ranged in age from 22 to 36 and two-thirds were women. Of the total, 193 were symptomatic and 110 were asymptomatic.

Seoul
People wearing face masks walk through an underground shopping area in Seoul on May 6 Photo: AFP / Jung Yeon-je

Among those who were initially asymptomatic, 89 never developed symptoms at all -- about 30 percent of the total.

This finding itself helps gives a sense of what portion of infected people are truly asymptomatic rather than merely "presymptomatic," a subject of confusion.

All were sampled at regular intervals after day eight of isolation, and the samples returned comparable values of the virus' genetic material from the upper and lower airways.

The median time taken for the patients to return negative tests was marginally less for asymptomatic patients compared with symptomatic: 17 and 19.5 days, respectively.

The authors wrote their findings "offer biological plausibility" to reports of asymptomatic transmission.

But they added that their study only looked at the amount of viral genetic material present and did not attempt to follow the subjects to see if that translated to the spread of infectious virus.

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