Of the 53 new cases reported, 50 are in Daegu, 300 kilometers southeast of Seoul, and neighboring North Gyeongsang Province. The remaining three were reported in Seoul, the KCDC said in a statement. Daegu and North Gyeongsang Province are home to some 5 million people.
Of the 50 new cases in Daegu, at least 30 patients are believed to be linked to a 61-year-old South Korean woman who was confirmed to be infected Tuesday, the KCDC said. The country's 31st virus patient attended church services in Daegu and went about for days before testing positive for the virus.
An epidemiological survey is also under way to determine whether or not the 31st patient is linked to two confirmed cases at the hospital in Cheongdo, south of Daegu, the KCDC said, adding the patient visited Cheongdo earlier this month.
The KCDC said at least 42 virus patients are presumed to be linked to the woman. KCDC Director Jung Eun-kyeong called for people who visited the Daegu church and the hospital in Cheongdo to stay indoors or report to health authorities if they show symptoms.
The number of coronavirus patients who are receiving oxygen therapy increased to four from one Thursday, Jung told reporters. Most new cases have no clear ties to travel to China, where the virus originated, or Southeast Asian countries, the KCDC said.
Jung said the 31st virus patient may have contracted the virus via other patient, indicating that there may be more infections. Authorities probed two worship services at the Daegu church, which were held on Feb 9 and 16, and attended by the 31st patient.
"Currently, we are investigating the possibility that the 31st patient contracted the virus via a secondary infection," Jung said.
The country's probable coronavirus "super spreader" was hospitalized at a Daegu hospital from Feb. 7 to Monday. The Shincheonji Church of Jesus in Daegu, where the 31st patient attended worship services, said in a statement that it had been shut down after about 10 members tested positive for the virus.
Local media reports said about 1,000 members attended worship with the 31st patient. The KCDC said 1,001 members of the church are asked to self-isolate to contain the spread of the virus. In a televised news conference, Daegu Mayor Kwon Young-jin urged the city's 2.5 million people to stay indoors after 35 new cases were reported in two days in the city and its neighboring areas.
The number of people being checked for the virus and under quarantine came to 1,860 on Thursday, up from 1,633 earlier in the day, the KCDC said. South Korea has screened 13,098 people for COVID-19 since Jan. 3, with 11,238 testing negative and 16 patients having been discharged from quarantine after making full recoveries.
South Korea also started to check those who show suspicious symptoms for the novel coronavirus as it grapples with new infections without links to existing cases or overseas travel. The measure broadens screening for more people and make it easier to detect and isolate COVID-19 cases, the KCDC said.
Under the latest revision, front-line doctors will be allowed to actively conduct coronavirus tests if they think a person may have contracted the sickness. South Korea has a capacity to conduct 5,000 coronavirus tests a day, but the figure will be increased to 10,000 by the end of the month.
In addition, the country plans to check those who are suffering from pneumonia due to unknown causes.