IFNG and COVID-19: However, a recent study demonstrated that, among SARS-CoV-2-specific CD8+ T cells detected by MHC-I multimer staining, PD-1+ cells, as well as PD-1− cells, produced IFN-γ in patients with COVID-19 regardless of disease severity, indicating that SARS-CoV-2-specific CD8+ T cells are not exhausted, but functional.5 Further studies are required to examine the functional characteristics of CD8+ T cells in the exhausted cluster identified by Kusnadi et al. In addition, further studies are required to reveal a possible association between co-morbidities of COVID-19 patients and T cell functions.