Four nurses at one of the largest hospitals in South Korea tested positive for the novel coronavirus in a new cluster of infections putting authorities on high alert again Tuesday.

The infections have sounded the alarm about asymptomatic patients, as two of the four nurses had not displayed symptoms of the virus.

The first case, involving a surgical nurse, was confirmed at Samsung Medical Center on Monday evening and three more nurses were diagnosed with COVID-19 early Tuesday, Seoul Mayor Park Won-soon said.

The first patient has been absent from work since Friday and fell sick with fever on Sunday, according to city and hospital officials.

“There is a possibility of more patients. … Given a cluster infection emerged at a large hospital where there are many patients in intensive care with respiratory problems, we need to respond carefully and sensitively,” Park said.

He added an 18-member response team was contact tracing and examining the transmission route among others.

Some 260 people who had come into contact with the nurse were tested, of which 160 tested negative, according to Gangnam District.

Test results for 100 people were pending, as of Tuesday morning, while around 10 people were waiting to be tested.

Medical staff who had contacted the nurse have been put under self-quarantine for two weeks, the hospital said.

The facility has temporarily closed some operating rooms, the hospital said.

“We apologize to our patients, who trust Samsung Medical Center, for the trouble the diagnosis of a coronavirus patient has caused you. … An epidemiological survey is underway. … We will do our best to return back to normal as soon as possible,” the hospital said in a statement.

There were some 1,980 hospital beds and roughly 3,700 physicians and nurses combined at the third-largest general hospital, as of late 2017, according to the hospital.

The nurse has not been to Seoul’s nightlife district, Itaewon, where a cluster of infections surfaced last week, local authorities said.

Touching on Moderna’s coronavirus vaccine, Korea Centers for Disease Control and Prevention Deputy Director Kwon Jun-wook said large-scale production of the vaccine could be on an accelerated timetable.

“Moderna has said it aims to mass-produce (the vaccine) in the first half of next year, but the schedule could be brought forward significantly,” Kwon said during an afternoon briefing on COVID-19.

He said vaccine development was also underway here centered on at least two companies and research institutes.

The infection of the surgical nurse has sparked health safety concerns amid respite from COVID-19.

The virus officially sickened 13 more people in Korea on Tuesday from the previous day, with no new deaths, according to the KCDC.

The number of daily new infections stayed in the tens for the fourth day in a row.

Counting the latest infections, the country’s case count stood at 11,078, while the death toll remained at 263, figures from the KCDC showed.

Nine of the new cases were local transmissions -- six from Seoul, two from Incheon and one from Gyeonggi Province –- and all four imported cases were reported out of Ulsan City, according to the emergency disaster control headquarters.

By Kim Bo-gyung (lisakim425@heraldcorp.com)