South Korea has confirmed five more cases of so-called breakthrough COVID-19 infections, bringing the total caseload here to nine, health authorities said Thursday.

The Korea Disease Control and Prevention Agency (KDCA) said all the breakthrough infections were in those who had two doses of the vaccine by US pharmaceutical giant Pfizer Inc.

A "breakthrough case" is when a person tests positive for COVID-19 between the first and second doses of a two-dose regimen or a person tests positive after full vaccination.

All of the five people are those aged between 70 and 80, with three of them being female and the two others male.

The KDCA said similar cases will increase as the country's vaccine rollout revs up, adding that those who are fully inoculated still need to wear masks.

The authorities said even after full vaccinations, a person can be infected with COVID-19. They also said that those who are classified as breakthrough infection cases have mild symptoms or are asymptomatic.

As of Thursday, a total of 6.74 million people, including 381,551 the previous day, have received their first shots of COVID-19 vaccines since the vaccine rollout on Feb. 26, taking up 13.1 percent of the total population, the KDCA said. (Yonhap)