South Korea’s new daily COVID-19 case numbers could soar to 400 in four weeks should the current infection rate continue, health authorities warned Monday, due to increased contact among people and the winter season keeping people in enclosed spaces.

The rapid spread of the virus across the country might be imminent due to virus carriers showing no or mild symptoms, most of them relatively young, the Korea Disease Control and Prevention Agency’s director, Jung Eun-kyeong, said at a briefing Monday.

Korea reported 223 new coronavirus cases -- 193 locally transmitted and 30 imported from overseas -- in the 24 hours ending Sunday at midnight, according to the agency.

The daily case count topped 200 for the third consecutive day Monday, seeing a triple-digit gain for nine days straight.

The R number -- the average number of secondary infections produced by a single infected person -- is about 1.1, according to Jung.

One of the major reasons behind the rise in infections is increased contact among people at social events, gatherings and activities following the easing of social distancing rules last month, she said.

“Especially when it comes to young people, they don’t get tested for the virus as they often show no symptoms or mild symptoms but their scope of social activities is very wide, which could heighten risks of silent spread of the virus,” she said, calling for more testing among the young.

More than half of the confirmed patients reported in the past week, or 52.2 percent, were under 50 years old. Over the recent four weeks, the proportion of patients who were under 50 was 49.1 percent, up 10.8 percentage points from the previous four weeks.

The proportion of cases with unknown transmission routes has increased to 14 percent for the past two weeks, according to the KDCA.

Another factor is the colder weather, she said, which leads to more activities in enclosed spaces with a lack of ventilation.

Clusters of infections continue to be reported in connection with everyday spaces such as cafes, restaurants, workplaces and indoor sports facilities, as well as hospitals and nursing homes.

The Greater Seoul area and Gangwon Province are on high alert, with health authorities consulting municipalities over whether to raise the level of social distancing by a notch from the current Level 1 to Level 1.5 there. The seven-day daily average of new cases was 99.4 in the Greater Seoul area and 13.9 in Gangwon Province, respectively, in the past week.

Regional Level 1 rules -- which are the least restrictive in the country’s five-tier distancing scheme -- are in place when the number of daily new cases stays below 100 in the Greater Seoul area; below 30 in the Chungcheong, Jeolla and Gyeongsang provinces; and below 10 in Gangwon Province and on Jeju Island.

Of Monday’s locally transmitted cases, the majority were in Greater Seoul -- 79 in Seoul, 39 in Gyeonggi Province, which surrounds the capital, and 10 in Incheon. Twenty cases were reported in Gangwon Province, 13 in North Gyeongsang Province and 10 in South Jeolla Province.

Korea is also grappling with a steady influx of imported cases from overseas as the coronavirus rages around the globe.

The country added 30 imported cases. Among them, 23 cases were detected at the border, while the other seven cases were identified while the individuals were under mandatory self-quarantine in Korea.

Twenty-four cases were from elsewhere in Asia, four from the United States, one from Germany and one from Bulgaria. Twenty of those cases involved foreign nationals.

The number of COVID-19 patients in serious or critical condition here was 55.

One more person died from the virus, bringing the death toll to 494. The overall fatality rate is now 1.72 percent.

So far, of the 28,769 people confirmed to have contracted the new coronavirus here, 25,759 have been released from quarantine upon making full recoveries, up 68 from a day earlier. Some 2,516 people are receiving medical treatment under quarantine. 

The country carried out 10,813 tests in the past day. A total of 38,314 people were awaiting results.

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