The pathogenesis of COVID-19 involves both humoral and cellular immunological responses, with cell-mediated immunity being discussed as the primary and most effective immune response to viral infection. It is supposed that COVID-19 vaccines also elicited effective cell immune response, and specifically IFNγ secreted by SARS-CoV-2-specific T-helper 1 and Tcytotoxic cells. Using an interferon-gamma release assay (IGRA) test, we aimed to monitor cellular post-vaccination immunity in healthy subjects vaccinated with BNT162b2 mRNA COVID-19 vaccine (Comirnaty). We tested 37 healthcare workers (mean age 54.3 years, range 28–72, 22 females, 15 males) following COVID-19 mRNA COVID-19 vaccine and 15 healthy unvaccinated native persons as control subjects using QuantiFERON SARS-CoV-2 RUO test, performed approximately 1 month after vaccination. We also measured virus-neutralizing antibodies. Thirty-one out of 37 tested subjects had significantly raised levels of SARS-CoV-2 specific IFNγ against SARS-CoV-2 Ag1 and Ag2 1 month following COVID-19 vaccination. In addition, we found a significant difference between the IFNγ levels in fully vaccinated subjects and the control group (p < 0.01).We also found a substantial correlation (r = 0.9; p < 0.01) between virus-neutralizing antibodies titers and IFNγ concentrations released by T cells. We believe that IGRA tests are an excellent tool to assess the development of a post-vaccination immune response when immunized against SARS-CoV-2. However, IGRA-based tests should be performed within a few weeks following vaccination. Therefore, we can speculate that the application of these tests to assess long-term immune response is debatable.