Indeed, COVID-19 patients infected by SARS-CoV-2 have been reported to have increased percentages of CD57+ PD-1+ CD4+ cells, which might increase throughout the infection [13, 14], and disrupted germinal center reactions with decreased BCL6-expressing Tfh and GC B cells and reduced total CD3+ T- and B-cell counts have also been reported in acute COVID-19 patients [82]. Here, PDCD1 is linked to COVID-19.