IL2 and COVID-19: As a further parameter of T-cell activity/T-cell response, we determined the cytokine production of SARS-CoV-2-specific T cells per single cell and observed no significant differences for spike S1- and S2-reactive IFN-γ- and IL-2-secreting cells between hoVac and heVac KTRs after second and third COVID-19 vaccination (Figure 2C and Additional file 2: Figure S2).