New virus cases in Korea rose back to 51 on Wednesday, as the nation saw continued local cluster infections coupled with the rebound in imported cases.

Korea failed to keep the daily tally below 50 on seven days this month, one of the criteria for maintaining relatively eased restrictions. As things stand now, the public health authorities will have to stage a fight on both local and foreign fronts for the time being, epidemiological experts said.

The 51 new cases -- 31 local infections and 20 foreign arrivals -- raised the cumulative caseload to 12,535 as of midnight Tuesday, the Korea Centers for Disease Control and Prevention (KCDC) said.

According to the KCDC, 22 more people have recovered, increasing the number of cured patients to 10,930, with 1,324 people receiving treatment. The country reported no new virus-linked death, keeping the death toll at 281. It has tested 1,208,597 suspected patients since Jan. 3. Among them, 1,175,817 people tested negative with the other 20,245 awaiting results.

Health officials confirmed 19 additional cases from the densely populated greater Seoul areas, where around half of the country's 50-million population lives, while Daejeon, a metropolis 160 km south of Seoul, added eight cases.

Despite intensive track and trace efforts, quarantine officials have failed to contain spreads from a door-to-door marketing company, Richway, which held sales events for elderly citizens.

The failure to block imported cases through ports has emerged as another problem, with most cases traced to sailors on a Russia-flagged ship docked in Busan, the second-largest city.

The incident has also highlighted the loophole in quarantine procedures at the local ports, as the ship was found to have submitted falsified electronic documents over its quarantine situation.

The operator of the ship did not inform local regulators that three of its sailors showed symptoms of high fever, while Russia, the place where the boat departed initially from, also did not notify Korea that the captain had tested positive for the virus a week earlier.

To contain further spread of the virus from the ship, health officials urgently quarantined over 170 people who worked in the vicinity of the vessel and planned to conduct Covid-19 tests on all isolated people.

Amid a continued rise in cluster infections, health authorities are considering expanding stricter infection preventive measures -- currently in place only in the Seoul metropolitan area -- across the country.

On a brighter side, the health authorities said that 185 fully recovered patients had donated plasma to help develop the Covid-19 blood plasma treatment, up 123 patients from June 6. The plasma treatment is a hyperimmune globulin made by dividing up only immune proteins with various antibodies taken from the plasma of recovered COVID-19 patients.

The treatment is highly promising, as it has the potential to be one of the earliest treatment options for COVID-19, officials said.

They requested more active participation from cured citizens as the blood plasma treatment requires a lot of blood to concentrate and formulate the antibodies and immunoglobulins.

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