Since March 2021, we have identified 4 breakthrough infections among 8678 fully vaccinated health care workers at Asan Medical Center. All 4 of the workers had received two doses of the ChAdOx1 nCoV-19 vaccine (Oxford–AstraZeneca) within a 12-week period. The median interval between the receipt of the second dose of vaccine and diagnosis was 37 days (range, 29 to 42), and the median cycle threshold (Ct) on the STANDARD M nCoV Real-Time Detection Kit (SD Biosensor) was 13.1 (range, 12.3 to 16.0).