CD28 and COVID-19: Beyond the antibody responses to COVID-19 vaccination, we also assessed the percentage of CD4 T cells and CD8 T cells not only naive (CD28+CD57-) but also double-positive (CD28+CD57+) and senescent (CD28-CD57+), before and 30 days after administration of the second dose of CoronaVac and ChadOx-1 vaccines.