A fourth round of COVID-19 vaccination is set to start on Monday for people with weak immune systems who have completed their third round of inoculation.

Authorities said local clinics will offer the second booster shots to around one-point-three million adults aged 18 and older who have preexisting health conditions or have weakened immunity from taking immunosuppressants.

Those eligible can seek same-day vaccination by making appointments via mobile apps or by contacting clinics directly. They can also make online reservations, with appointments available starting February 28. The group will be provided with either the Pfizer or Moderna vaccine.

Around 500-thousand patients under care at nursing facilities and employees will also receive the fourth dose from the first week of March. They will be eligible for the shot four months after their first booster.

Authorities said the second booster shots are aimed at minimizing critical cases and fatalities amid the rapid spread of the omicron variant.