PRF1 and COVID-19: In addition, analysis of antigen-specific CD8+ T cells using HLA tetramers reveals that GZMB and perforin expression levels decrease with time after COVID-19 recovery (63), and a study based on serum levels rather than intracellular expression analysis revealed that GZMA and perforin serum levels are higher in SARS-CoV-2-infected patients with mild symptoms than in healthy individuals and critically-ill patients (64).