ABSTRACT
Since February 2021, healthcare workers in Japan have been preferentially vaccinated with a messenger RNA vaccine (BNT162b2/Pfizer) against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). While many studies have confirmed that this vaccine is highly effective in reducing hospitalizations and deaths from coronavirus disease 2019 (COVID-19), antibody titers tend to decline at 3 months, leading to a risk of breakthrough infections. Thus, information is needed to support decision making regarding the third vaccination. In this study, we investigated transition of the anti-SARS-CoV-2 spike protein receptor-binding domain (RBD) IgG and neutralizing antibody titers of 37 vaccinated Japanese healthcare workers. Samples were collected six times starting prevaccination until 6 months after the second vaccination. Anti-SARS-CoV-2 RBD IgG levels peaked at 1 week after the second vaccination, then declined over time and decreased to <10% at 6 months after the second vaccination. Additionally, approximately one third of subjects at 6 months after the second vaccination were seronegative for the Omicron variant. Workers with low anti-SARS-CoV-2 RBD IgG levels also had low neutralizing antibody titers. These data support the active use of boosters for healthcare workers, especially for those with low anti-SARS-CoV-2 RBD IgG levels.