We studied three ostensibly healthy healthcare workers who received a single booster of the novel Pfizer/Biontech mRNA BNT162b2 bivalent vaccine in February 2023. Blood was drawn immediately before vaccine injection (i.e., D0) and at fixed times afterward (after 1 [D1], 4 [D4], 5 [D5], 7 [D7] and 14 [D14] days). ). Total anti-SARS-CoV-2 serum antibodies were assayed with Roche Elecsys Anti-SARS-CoV-2 electrochemiluminescence immunoassay, whilst cellular immunity was quantified using the Elecsys IGRA SARS-CoV-2 test. The value of IGRA SARS-CoV-2 started to increase already at day 1 after COVID-19 bivalent vaccination, peaked at day 4 (5.1±3.4 folds increase), but then sharply declined already at day 5, then stabilizing at values around 2.6-2.9 folds higher than the baseline in the following period. The value of total anti-SARS-CoV-2 antibodies started to increase at day 5 after vaccination, and then continue to gradually increase up to the end of the observational period (i.e., plateauing at 3.6±1.6 folds increase at day 14). These findings provide evidence to support the clinical observation that COVID-19 vaccines administered within a few days after SARS-CoV-2 exposure may be effective in halving the risk of developing severe COVID-19 illness, whereby we showed that vaccine efficacy in boosting T cell response begins to manifest very early (i.e., within few days) after administration.
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