The health authorities said Tuesday that nearly half of the confirmed Covid-19 cases in the past two weeks were caused by breakthrough infections.

According to the Central Disaster and Safety Countermeasures Headquarters, out of the 17,325 people who tested positive for the virus over the past two weeks (Oct. 17-30), vaccinated people accounted for the most cases with 8,336 people or 48.1 percent. Some 32.8 percent of them had received no shots, and 19.1 percent had taken their first jab.

The data showed that the rate of breakthrough infection increased with age.

Only 19.7 percent of newly confirmed cases aged 18-29 got breakthrough infections. In contrast, 84.4 percent of new patients in their 70s were due to breakthrough infections. The comparable rates for other age groups were 81 percent for the 60s, 74.6 percent for the 80s, and 60.2 percent for the 50s.

Aside from breakthrough cases, health authorities expressed concerns about the increase in the number of confirmed cases among those under 18, especially middle and high school students aged 13 to 17.

The weekly average Covid-19 case rate of 13-17-year-olds was 8.5, significantly higher than the average of 6.3 patients among those aged 10-19.

Quarantine officials attributed these mid-teen patients to schools, PC cafes, and coin karaoke rooms, pointing out that many did not follow basic quarantine rules, such as wearing masks at school.

"We will take measures to prevent infection among teenagers ahead of this month's College Scholastic Ability Test (CSAT)," health officials said. "Also, the government will provide information about the safety and effectiveness of vaccination to schools and parents to increase the vaccination rate for children and adolescents."

The government has vaccinated 41,638,366 people – 11,110,862 with the AstraZeneca vaccine, 22,320,645 with Pfizer's vaccine, 1,489,712 with Janssen's vaccine, 6,601,460 with Moderna's vaccine – with the first shot of the vaccine up 115,687 from the previous day.

As of Tuesday, 76.9 percent of Koreans were fully vaccinated, while 81.1 percent have received their first shots.

Korea's new coronavirus cases stayed below 2,000 for the second straight day Tuesday, but health authorities remained on alert over a possible spike in new infections under eased virus curbs.

The Korea Disease Control and Prevention Agency (KDCA) reported 1,715 new virus cases, including 1,698 local infections, raising the cumulative caseload to 383,407 as of midnight Tuesday. Eighteen more people died from the virus, raising the death toll to 2,998 with a fatality rate of 0.78 percent.

Korea has kicked off the first of the three-stage "living-with-Covid" scheme to gradually phase out quarantine restrictions as more than 70 percent of the population completed their vaccination.

Under the latest measure, effective for four weeks, it will lift all restrictions on the business hours of cafes, restaurants, and other businesses, except for nightlife establishments. It also raised the private gathering limit to 10 people for the greater Seoul area.

However, it will strictly reinforce the vaccine pass system on high-risk facilities, such as indoor sports facilities, bars, and nightclubs. The vaccine pass requires visitors of such multi-use facilities to have a vaccination certificate or negative test result.

The government also said that it would soon release the quarantine indicators to judge the transition to the next stage "living-with-Covid" scheme or maintain the current stage, and the review and implementation of the emergency plan.

The authorities planned to reveal such information on Tuesday but postponed it, accepting the opinions of related ministries and other experts.

"When an index is set, it becomes a reference point, and rationale for judgment, having huge social impact," KDCA's chief epidemiologist Lee Sang-won said. "Therefore, we had to delay the announcement as we needed more time to judge whether there are any loopholes or whether we have sufficiently considered the influence of the index."

In setting the management index, the key is to know how many medical resources are available and whether we can manage critical patients and lower mortality, Lee added.

Lee stressed that the most important factors are the intensive care unit utilization rate and patient bed capacity.

"Instead of making a judgment based on a single indicator, the government will determine the stage of daily recovery based on experts’ discussion and overall situations," Lee added.