South Korea has confirmed four cases of what's called a breakthrough COVID-19 infection where a fully vaccinated person tests positive for the virus.

The Korea Disease Control and Prevention Agency(KCDA) said Tuesday that the four cases were confirmed as of 12 a.m. last Friday. That’s about point-0003 percent of some one-point-48 million people who’ve been fully vaccinated in the country. 

The agency defines as breakthrough cases as patients who become infected with COVID-19 even after receiving the two full doses of a vaccine in line with recommended standards. 

According to the agency, epidemiological survey results assessed that two of the four people were likely to have been exposed to the virus before two weeks passed after getting their second shot.

The U.S.’ Centers for Disease Control and Prevention defines a breakthrough case as an infection detected more than 14 days after receiving both doses of the COVID-19 vaccine.