Korea reported 20 new coronavirus cases Friday amid efforts to prevent further cluster infections, which started at some bars and clubs in Seoul and spread to various parts of the country.

Among the 20 newly confirmed patients, 11 cases were local infections. The total number of confirmed caseload reached 11,142, the Korea Centers for Disease Control and Prevention said. The death toll stood at 264 as of midnight Thursday. Twenty-seven more patients recovered to push up the total to 10,162. The nation, with a population of 50 million, has tested more than 800,000 people.

The health officials were fretting about asymptomatic patients who can spread the virus without showing any symptoms. Infectious diseases generally have infectivity after the patient show develop symptoms. Still, in the case of Covid-19, the virus starts transmission even before showing symptoms.

A substantial part of the virus, which spread from clubs in Itaewon, Seoul, to pubs, karaoke rooms, and private schooling facilities, is related to asymptomatic patients.

Infected people without symptoms are responsible for a chain infection by going ahead with their daily routines, and finding these patients takes time due to late testing or none at all. This is why they are called the “silent spreaders, according to epidemiological experts.

The public health authorities also said they believe that community infections began before there appeared four confirmed cases among the nurses at Samsung Medical Center on Monday. Health officials stressed that the public should follow personal hygiene and everyday life quarantine measure as asymptomatic patients are challenging to find.

In the same vein, Minister of Health and Welfare Park Neung-hoo asked the people to refrain from using multiuse facilities, including karaoke and internet cafes, near schools. “The latest cluster infections, which began in some nightlife districts in Seoul, continue to infect people sporadically through clubs, taverns, and karaoke rooms,” Park said.

Dozens of high schools in Gyeonggi Province that surrounds Seoul ordered students to return home on Wednesday. Two high school students in Incheon tested positive for Covid-19, in what could be infections tied to Itaewon.

Another vocational school in Daegu, once the epicenter of the new coronavirus pandemic in Korea, also closed down on Thursday after a student in a dormitory tested positive.

The nation, which usually starts the new school year in March, allowed high school seniors to return to classrooms on Wednesday, to have students of other grades gradually resume regular routines by early June.

Meanwhile, the police said they had put a Japanese young man, 23, under arrest for failing to abide by the stay-at-home rule and went out of his residence several times. The man, 23, was the first foreigner who has been arrested for violating the self-quarantine system.

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