South Korea confirmed 2,434 new cases of the coronavirus on Thursday -- a record high for the country.
The South Korean government had eased some restrictions on gatherings during the annual holidays earlier this week.
Family gatherings of up to eight were allowed if they included fully vaccinated people.
Health authorities said cases could rise further after people who traveled during the holidays are tested for the coronavirus and receive the results next week.
They are urging the public to take virus tests, to stay home and avoid gatherings, and to get vaccinated.
Prime Minister Kim Boo-kyum instructed authorities to monitor the infection situation, saying the impact of the holiday period will become clearer next week.