President Moon Jae-in says South Korea will hold off going to the next stage of a return to normal life.
Instead of easing restrictions further, there'll be special measures taken for the next four weeks to prevent infections and treat those who do get infected.
Moon chaired a special meeting Monday on the nation's COVID-19 response.
"We cannot let the difficult start we've had in the gradual return to normal life lead to a retreat to the past. The key to the special measures is vaccination. It has become clear that high efficacy can be maintained only after the third shot. We must now acknowledge that the third shot is no longer an additional dose, but a requirement for full vaccination."
South Korea started easing its restrictions as part of a gradual return to normal life in early November.
Moon stressed that it was important to speed up vaccinations among teenagers, calling on officials to look at what's been done in other countries.
The president also called on the government to ensure there is no vacuum in the treatment of patients and to swiftly secure oral treatments ahead of schedule, which is currently February next year.
He also called for stronger tracking, tracing and containment measures to prevent the local transmission of the new variant omicron.