In 16 COVID-19 patients, comparison of mild and severe cases of disease identified CD4+ T cell functional defects and CD8+ T cell exhaustion associated with severe outcome (10); lower expression of TNFα and IFNγ producing CD4+ T cells and higher levels of killer CD8+ T cells expressing Granzyme B and perforin were present in severe cases. This evidence concerns the gene PRF1 and COVID-19.