Transmission routes were unknown for 1 in 5 coronavirus cases reported over the past two weeks in South Korea as of Tuesday, signaling a tough battle in reining in the spread of the virus as the world watches COVID-19 vaccinations begin in the UK.

Korea added 594 coronavirus cases -- 566 locally transmitted and 28 imported from overseas -- in the 24 hours ending Monday at midnight, according to the Korea Disease Control and Prevention Agency.

For the past two weeks, transmission routes for 20.7 percent of the cases were unknown as of Tuesday, exceeding the 20 percent mark for the first time in 70 days, according to the KDCA.

Tuesday’s daily case count fell below 600 after the country reported 615 cases Monday and 631 cases Sunday, with the country having seen triple-digit gains in infections since Nov. 8. The seven-day average number of coronavirus cases from Dec. 2-8 was 586.1.

New clusters of infections emerged from several pubs where people play poker in central Seoul, with a total of 19 cases linked to the pubs so far. Another cluster was traced to a market in central Seoul, with 14 cases reported so far. In connection with a restaurant in Busan, 16 people tested positive for the virus.

Cases linked to a restaurant and a singing class in central Seoul surged by 51, bringing the total of related cases to 162. 

Of Tuesday’s locally transmitted cases, 65 percent were in Greater Seoul -- 212 in Seoul, 146 in Gyeonggi Province, which surrounds the capital, and 27 in Incheon. Outside the Seoul metropolitan area, 61 cases were registered in Ulsan due to the rising number of cases linked to a nursing home, and 25 cases were posted in Busan.

Tougher antivirus restrictions went into effect Tuesday nationwide -- Level 2.5 social distancing measures in the Seoul metropolitan area and Level 2 outside the region. They will be in place until Dec. 28 at midnight.

Under Level 2.5 restrictions, the second-toughest in the country’s five-tier social distancing system, gatherings of more than 50 people are banned, indoor sports facilities cannot operate and establishments such as malls, stores, internet cafes and theaters must be closed after 9 p.m. These are in addition to Level 2 restrictions such as banning indoor dining at cafes all day and at restaurants after 9 p.m.

Of the 28 imported cases reported Tuesday, seven were detected at the border, while the other 21 were identified in individuals under mandatory self-quarantine in Korea. Seven cases were from elsewhere in Asia, 16 were from the United States and one each came from Mexico, Switzerland, Australia, Tanzania and Nigeria.

Sixteen of those cases involved foreign nationals.

The number of COVID-19 patients in serious or critical condition here stood at 134.

Three more people died from the virus, bringing the death toll to 552, with the overall fatality rate at 1.42 percent. 

So far, of the 38,755 people confirmed to have contracted the new coronavirus here, 29,650 have been released from quarantine upon making full recoveries, up 349 from a day earlier. Some 8,553 people are receiving medical treatment while under quarantine.

The country carried out 11,949 tests in the past day. A total of 71,274 people were awaiting results.

Meanwhile, people in the UK began to receive the first doses of coronavirus vaccines manufactured by the Pfizer and BioNTech, a week after the country became the first Western nation to authorize widespread use of a vaccine against COVID-19.

In Korea, the government announced Tuesday it had secured coronavirus vaccines for 44 million people and it will start administering the vaccine in stages starting in February for populations that are at high risk of infection.

By Ock Hyun-ju (laeticia.ock@heraldcorp.com)