Enhancement of humoral and cellular immunity against SARS-CoV-2 by a third dose of BNT162b2 vaccine in Japanese healthcare workers

The Omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has raised concerns regarding vaccine effectiveness. We investigated humoral and cellular immune responses against SARS-CoV-2 in healthcare workers before and after a third (booster) dose of the BNT162b2 mRNA vaccine. It significantly enhanced both humoral and cellular immunity in previously uninfected individuals. However, cellular immunity was not enhanced in previously infected persons, suggesting that three antigenic stimuli by vaccination or natural infection reached a plateau of cellular immunity. Even with reinforced immunity to SARS-CoV-2, we confirmed several post-booster breakthrough cases caused by the Omicron variant.