Six people were confirmed to have been infected with COVID-19 after getting their first doses of vaccines, which have been administered to health care workers and patients at long-term nursing facilities since late last month, the health authority said Tuesday.

The Korea Disease Control and Prevention Agency (KDCA) said three more tested positive for the coronavirus after their first vaccine shots, in addition to three cases confirmed Monday.

Two nurses and a medical employee working at the National Medical Center were confirmed to have been infected with the virus Monday after receiving their first doses of Pfizer vaccine shots last week.

Health authorities stressed that the virus infections are not related to the vaccine's efficacy as it takes at least two weeks for antibodies to be formed after vaccination.

"They may have been already infected (with the virus) before the inoculation or got infected before antibodies were formed (after the first jab)," the KDCA said.

The country has been administering two vaccines, from AstraZeneca and Pfizer, which require two shots to be effective, since late February to front-line health officials and health care workers in nursing homes.

The AstraZeneca vaccine is currently not available for those aged 65 and over, but health authorities are widely expected to approve the use of the AstraZeneca vaccine for the age group.

A total of 383,346 people have been inoculated since the first shot was administered on Feb. 26, according to the KDCA.

AstraZeneca's vaccine has been given to 377,138 people, while 6,208 received that of Pfizer. (Yonhap)