Breakthrough infection (BI) after coronavirus disease 2019 (COVID-19) vaccination has exploded owing to the emergence of various SARS-CoV-2 variants and has become a major problem at present. In this study, we analyzed the epidemiological information and possession status of neutralizing antibodies in patients with BI using SARS-CoV-2 pseudotyped viruses (SARS-CoV-2pv). Analysis of 44 specimens diagnosed with COVID-19 after two or more vaccinations showed high inhibition of infection by 90% or more against the Wuhan strain and the Alpha and Delta variants of pseudotyped viruses in 40 specimens. In contrast, almost no neutralizing activity was observed against the Omicron BA.1 variant. Many cases without neutralizing activity or BI were immunosuppressed individuals. The results of this study show that BI occurs even when there are sufficient neutralizing antibodies in the blood due to exposure to close contacts at the time of infection. Thus, even after vaccination, sufficient precautions must be taken to prevent infection.
