IFNG and COVID-19: Recent reports have shown that IFN-γ-producing cells are lower among SARS-CoV-2-specific CD8+ T cells than those specific to influenza A virus; however, the proportion of IFN-γ-producing cells is mainly within the fraction of PD-1+ cells, suggesting that in the COVID-19 context, PD-1-expressing CD8+ T cells are not exhausted and these are functional cells [58].