IL21 and COVID-19: Here, we found that after a third or fourth COVID-19 vaccination, there was a significant increase in IL-21 memory T-cell and B-cell responses in KTR whose humoral response was impaired, although these responses remained lower than those observed in healthy individuals after 2 primary mRNA-1273 vaccinations.8 Although IFN-γ producing T cells represent the current “gold standard” for assessing cellular responses to COVID-19 vaccination, IFN-γ responses did not increase following repeated vaccination, in contrast to IL-21 responses.