Abstract
BACKGROUND/AIMS: The rapidity of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific memory B or T cell response in vaccinated individuals is important for our understanding of immunopathogenesis of coronavirus disease 2019 (COVID-19). We therefore compared the timing of adequate immune responses between the first and booster doses of COVID-19 vaccines in infection-naïve healthcare workers. METHODS: We enrolled healthcare workers who received two doses of either the BNT162b2 vaccine or the ChAdOx1 vaccine, all of whom received the BNT162b2 vaccine as the booster (the third) dose. Spike 1 (S1)-immunoglobulin G (IgG) antibodies and interferon gamma producing T cell responses were measured at 0, 7, 14, and 21 days after the first dose, and at 0 and between 2 to 7 days after the booster dose. RESULTS: After the first-dose vaccination, the S1-IgG antibody responses were elicited within 14 days in the BNT162b2 group and within 21 days in the ChAdOx1 group. After the booster dose, the S1-IgG antibody responses were elicited within 5 days in both groups. The SARS-CoV-2-specific T cell responses appeared at 7 days after the first dose and at 4 days after the booster dose. CONCLUSION: SARS-CoV-2-specific immune responses by memory B cells and T cells may be expected to appear around 4 to 5 days after the booster dose.