Covid-19 Breakthrough Infections in Vaccinated Health Care Workers

Abstract

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).

On the basis of epidemiologic investigation and contact tracing, we determined that three of the workers had been infected by exposure to unvaccinated household members, and the remaining worker (HCW B) had been infected by another vaccinated health care worker (HCW A), who in turn had contracted a breakthrough infection from her unvaccinated spouse. (HCWs A and B had eaten a meal together and talked without wearing a mask for 20 minutes 1 day before the infection was diagnosed in HCW A; 5 days later, symptoms developed in HCW B.) PCR analysis with the use of the Kaira COVID-19 Variant Detection Kit (Optolane) revealed the presence of either the eta variant or the zeta variant (without sensitivity for further differentiation) in both health care workers. Although in our hospital breakthrough cases have been rare, viral loads among these cases have been moderate. Thus, our findings show that the transmission of SARS-CoV-2 between vaccinated persons is possible.