South Korea's daily new virus cases fell back to the 500s again Thursday as fewer people took virus tests on the Children's Day holiday, but the country's health authorities remained vigilant over cluster infections and spreading variant cases.

The country reported 574 more COVID-19 cases, including 562 local infections, raising the total caseload to 125,519, the Korea Disease Control and Prevention Agency (KDCA) said.

The daily caseload reached 488 on Monday and 541 on Tuesday but jumped to the 600s on Wednesday. Children's Day fell on Wednesday, and the daily tally is announced the day after.

The country added four more deaths, raising the death toll to 1,851, the KDCA said.

Untraceable cases accounted for 27.6 percent, or 2,443, of 8,858 confirmed cases during the period from April 23 to Thursday.

The country's health authorities are concerned that the more transmissible coronavirus cases are spreading in the southern part of the country.

On Wednesday, health authorities advised all employees at multiuse facilities in the southeastern city of Ulsan to take novel coronavirus tests following an alarming number of variant cases in the region. The move comes after 51 out of 80 new cases reported between March and the second week of April were confirmed to be variants that first originated in Britain.

The potential surge in mutated cases could hamper the country's virus battle, which has been dogged by continued sporadic cluster infections amid the vaccine rollout.

With an aim of achieving herd immunity by November, the authorities are accelerating the vaccination campaign.

People aged 70-74 can make reservations for inoculations with AstraZeneca's vaccine from 10 a.m. Thursday to June 3.

A total of 3,560,324 people, including 5,253 the previous day, have received at least their first shots of COVID-19 vaccines, taking up 6.9 percent of the country's population of 51.4 million, since the country started its vaccination program on Feb. 26.

AstraZeneca's vaccine has been given to 1.92 million people, while 1.64 million have received that of Pfizer.

A total of 18,260 cases of side effects after vaccinations have been reported, up 150 from a day earlier.

A total of 88 post-vaccination deaths have been reported, although the exact causes of the deaths remain unknown as they could not determine causality, the health authorities said.

In a briefing, the health authorities said enough vaccines have been secured, including 828,000 doses of Pfizer vaccine and 304,000 doses of AstarZeneca's, to inoculate 1.13 million people.

With an four additional inoculation centers, a total of 261 centers will be in operation across the country, they said.

"The new coronavirus cannot be vanquished with vaccine shots, and we can see the pathogen every year," KDCA chief Jeong Eun-kyeong said. She said mutated strains of the virus can lead to another wave of the pandemic.

Of the locally transmitted cases, Seoul reported 211 new cases, and Gyeonggi Province that surrounds the capital city identified

142 new patients.

Gangwon Province registered 12 additional cases, South Gyeongsang Province 17 cases, and Busan and Ulsan, major cities in the southeastern region, reported 28 and 38 new cases, respectively.

There were 12 new imported cases, down 13 from a day earlier, the KDCA said. The number of patients with serious symptoms across the country reached 172, down one compared with the previous day.

The total number of people released from quarantine after making full recoveries was 115,491, up 547 from a day earlier.

As of 6 p.m. Friday, health authorities and local governments had reported 576 new COVID-19 cases, up 174 from the same time Thursday.

Daily cases, compiled by health authorities and local governments, are counted until midnight and announced the following morning. (Yonhap)