Second COVID-19 booster shots for people aged 50 and older began Monday as the country battles a resurgence in cases.

The second boosters are also available for those aged 18 and older who have underlying health conditions, along with patients and workers at some facilities considered vulnerable to disease transmission, such as senior care centers.

For people aged 50 and older, the government will send out text messages to inform them that they are eligible for booster shots.

The government currently plans to administer COVID-19 boosters to people who had their last booster at least 120 days ago. But those who have personal reasons, such as leaving the country or planned hospitalization, could receive the booster 90 days after they had their last booster shots.

Individuals with a history of COVID-19 infections can also get their booster shot if 90 days have passed since their infection. However, health authorities recommend people to get their booster 120 days after their infections.

Starting on Monday, those who are eligible for COVID-19 booster shots can either look up leftover vaccines or make reservations. People can use the reservation platforms provided by KakaoTalk and Naver, or call local clinics or hospitals to put be put on a waiting list.

Although mRNA vaccines by Pfizer and Moderna are recommended, people can also choose to receive the Novavax COVID-19 vaccine.

Inoculations for those who make reservations will begin on Aug. 1.

Previously, only people aged 60 and older and people with an immune disorder were eligible for COVID-19 boosters. However, to tackle the rapidly spreading pandemic, the government has decided to expand its COVID-19 booster program.

On Monday, health authorities reported that the country’s daily COVID-19 infections continued to double on Sunday to reach 26,299, due to the highly contagious omicron subvariant BA.5.

The figure was down from 40,342 infections recorded on Saturday -- mainly due to fewer COVID-19 tests -- but it still marked a twofold increase from 12,681 cases posted a week ago.

The number of critically ill patients came to 81, up from the previous day’s 71. The number of deaths from the virus reached 11, down from the previous day’s 14.