Inflammatory factors induced by viruses can trigger a storm of mediators that cause changes in the differentiation and activation of T cells, disturbing the homeostasis of the immune system. In patients with COVID-19, the overall percentage of T lymphocytes is generally reduced, especially CD4+ CD3+ T lymphocytes, which have an activation phenotype, a reduction much more pronounced in severely ill patients. Furthermore, a higher percentage of CD4+ CD45RA+ naïve cells and lower CD4+ CD3+ CD45RO+ memory T cells were also found in COVID-19 patients (10, 153, 183, 184).