CD38 and COVID-19: CD8+ T cells in Post-COVID-19 furthermore showed upregulated expression of activation markers such as CD38 and HLA-DR (Fig. 3f) and higher PRF1 expression levels encoding the cytotoxic effector molecule perforin than Non-COVID-19 and Post-Vaccination (Fig. 3g), suggesting an increased cytotoxic potential of CD8+ T cells in hearts of Post-COVID-19 patients.