In their article on breakthrough infections among health care workers who had been fully vaccinated against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, Bergwerk and colleagues (published online on July 28)1 report that among 1497 such workers for whom polymerase-chain-reaction (PCR) data were available, 39 breakthrough infections were documented at their facility. There were no cases of secondary transmission among 31 of the workers with breakthrough cases who had available data regarding household transmission.

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).