Antibody tests on young enlistees in the nation’s military showed immunity levels three times higher than average for all age groups, government data revealed Monday.

Of the 6,959 enlisted soldiers aged between 20 and 22, whose samples were collected in September and October, 15 tested positive without having been previously diagnosed with COVID-19. Ten others with a history of COVID-19 also tested positive. This means 0.22 percent of the people considered uninfected had an immune response to the virus.

The figure compares to 0.07 percent immunity levels found in the third antibody tests conducted upon 1,379 ordinary citizens with no record of COVID-19.

Only one person from the National Nutrition Survey tested positive, according to the Korea Disease Control and Prevention Agency. The third-round test was based on the nutrition survey, which used samples collected from Koreans from 15 municipalities from Aug. 14-Oct. 31.

The result showed that infections that had not been previously detected -- mainly because the individuals showed few or no symptoms -- were more prevalent among the younger population, which authorities said means they carry high risks of spreading the virus in the local community.

The combined antibody rate from both of the tests was 0.19 percent.

“Even though the younger population get infected, they show no or mild symptoms and they actively lead social lives without being tested (for the virus) or getting a checkup from medical institutions, which poses considerably high risks of spreading the virus in the local community,” the KDCA chief Jung Eun-kyeong said at a briefing Monday.

“There needs to be toughened antivirus measures regarding those in their early 20s,” she said.

The antibody rate of 0.19 percent is extremely low, compared with other nations and global cities. New York City had a rate of 24.7 percent, London had 17 percent and Stockholm had 7.3 percent.

The results come amid growing concerns over a possible nationwide spread of the coronavirus due to increased contact among people and continued silent spread of the virus through carriers showing no or mild symptoms.

The country reported 271 new coronavirus cases -- 255 locally transmitted and 16 imported from overseas -- in the 24 hours ending Sunday at midnight, according to the Korea Disease Control and Prevention Agency.

It was the first daily tally below 300 in six days, but officials noted Monday that it was likely due to fewer COVID-19 tests being conducted over the weekend.

“The third wave of the pandemic is beginning due to the silent spread of the virus in our daily lives, which led to over 2,000 additional cases over the past week, as the weather gets colder,” Second Vice Health Minister Kang Do-tae said in a meeting with senior health officials.

“If we fail to break the chain of infections, antivirus measures and a proper medical response will not be sustainable,” he said.

Unlike in the first and second waves of infections earlier this year, most of which were traced to a few clusters, the third wave is being driven by multiple, sporadic clusters of infections, which complicates health authorities’ tracing and testing efforts.

The seven-day daily average of new cases stood at 255.6 over the past week, nearly double the 133.2 from a week earlier, the KDCA said. The proportion of cases with unknown transmission routes rose to 14.5 percent for the past week, up from 13.1 percent the previous week.

Tougher social distancing rules -- Level 2 under the country’s five-tier system -- are to take effect in the Greater Seoul area for two weeks starting Tuesday, with a size of gathering limited, dine-in services not to be allowed all day at cafes and after 9 p.m. at restaurants and bars, among other measures.

Of Monday’s locally transmitted cases, the majority were in Greater Seoul – 109 in Seoul, 74 in Gyeonggi Province, which surrounds the capital, and 23 in Incheon. Outside the Seoul metropolitan area, virus cases were registered in all parts of the country except for North Chungcheong Province.

Before Monday, Korea’s daily case count had exceeded 300 for five days straight, after staying in the 200-range for the previous four days. The daily case numbers have been in the triple digits since Nov. 8.

Out of the 16 imported cases reported Monday, five were detected at the border, while the other 11 were identified as the individuals were under mandatory self-quarantine in Korea. Five cases were from elsewhere in Asia, five from Europe, three from the Americas and three from Africa.

Six of those cases involved foreign nationals.

The number of COVID-19 patients in serious or critical condition here stood at 79.

Four more people died from the virus, bringing the death toll to 509, with the overall fatality rate at 1.64 percent.

So far, of the 31,004 people confirmed to have contracted the new coronavirus here, 26,539 have been released from quarantine upon making full recoveries, up 73 from a day earlier. Some 3,956 people are receiving medical treatment while under quarantine.

The country carried out 13,245 tests in the past day. A total of 56,455 people were awaiting results.

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