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People relax in Seoul’s Yeouido Hangang Park on Sept. 13. Access to many areas of the park has been restricted to limit the spread of COVID-19. (Yonhap News)

Antibodies capable of neutralizing the COVID-19 virus and preventing reinfection were found in just one blood serum specimen out of 1,440 collected throughout South Korea. The rate, which is much lower than the 10% range found in other countries, possibly indicates that infection clusters have not been taking place in a widespread manner thanks to the implementation of social distancing measures.

According to an announcement by the Korea Disease Control and Prevention Agency (KDCA), general and neutralizing antibody testing of 1,440 serum specimens collected from 13 metropolitan cities and provinces between June 10 and Aug. 13 for a national health and nutrition survey showed a positive result for only one specimen, which was collected in Seoul. This means that the general and neutralizing antibody possession rate for COVID-19 in South Korea amounted to just 0.07% at the time the study was conducted. Antibodies are typically formed in the bodies of people who contract infectious diseases; antibodies with response capabilities for a specific virus are known as neutralizing antibodies.

The results were much the same as those found in two past antibody possession rate studies conducted by disease control authorities. Another study of 1,500 serum specimens collected between May 25 and 28 from patients hospitalized at healthcare institutions in southwestern Seoul showed just one case in which neutralizing antibodies were confirmed. An initial analysis of 1,555 national health and nutrition survey specimens between Apr. 21 and June 16 did not find a single case of general or neutralizing antibodies. Some speculated at the time that this may have been due to the absence of serum from the hard-hit Daegu region in the sample -- but the latest study showed no antibodies in any of the 145 samples (10.1%) taken from the region.

According to some analysts, the reason so few neutralizing antibodies were found is because the virus’s transmission has been largely contained through social distancing and other disease control measures. KDCA Director Jung Eun-kyeong explained, “The fact that the positive rate [for antibodies] is not high is because community transmission has not been as widespread as it has been in Europe or the US.”

“This is the result of voluntary social distancing and everyday quarantine practices by the South Korean public,” she suggested. Previous studies have shown antibody possession rates of 3.2% in Wuhan, 14.9% in New York state, 17% in London and surrounding areas, 2-10% across Spain, and 7.3% in Stockholm.

Antibody possession rates serve as an indicator showing how many “hidden spreaders” a region has. But Jung cautioned, “These are the results of a study of around 1,500 serum specimens, and there are limits to our ability to generalize about the asymptomatic transmission rate [due to the small sample size].”

Regarding the relatively small percentage of hidden spreaders at a time when well over 20% of patients have no identified transmission route, Jung explained, “These were not samples from a period where we could observe the outbreak since August [when the number of confirmed cases increased].”

Disease control authorities plan to expand their study efforts by continuing to test remaining national health and nutrition survey serum specimens to determine the actual scale of hidden infections. Authorities are also planning to test 3,300 people from Daegu and North Gyeongsang Province, 10,000 recent conscripts, and a 10,000-person representative regional sample.

The low rate of antibody possession also means that so-called “herd immunity” is out of the question. Experts believe that herd immunity -- which allows for the containment of an infectious disease without the need for a vaccine -- can only be achieved when around 60% of a population possesses antibodies. The latest survey shows that South Korea is nowhere near that rate.

“This reaffirmed that herd immunity is not a viable disease control strategy available to South Korea,” said Lee Jae-kap, a professor of infectious diseases at the Hallym University medical school.

“We’re going to need to weather this by controlling the outbreaks through social distancing and other measures until a vaccine is produced,” he predicted.

By Choi Ha-yan, staff reporter

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