The first two confirmed cases of the Delta Plus variant in the country were identified as breakthrough infections with both patients having received the AstraZeneca vaccine before contracting COVID-19.

In a Tuesday briefing, senior health official Park Young-joon said the cases qualify since both patients tested positive two weeks after their second shots.

Authorities are on high alert over how the Delta plus, derived from the highly transmissible Delta strain, could impact the country's ongoing fourth wave of the outbreak.

The first case is a man in his 40s who has no overseas travel history. His son, who lives with him, has also tested positive for COVID-19 though it’s not yet known if he too has the subvariant. The second case came from overseas, however the person was fully vaccinated before traveling to the U.S.

Authorities said that the matter requires further examination.