CTLA4 and COVID-19: The frequency of CTLA-4+ Treg among Mild Recovered volunteers between those who had and had not experienced certain symptoms during acute COVID-19 was compared, and we observed that those who had not experienced sore throat, nasal obstruction, diarrhea, and ageusia had a lower Pool Spike CoV-2 peptide-induced CTLA-4 expression in Tregs compared with that in unstimulated cells (Figure 4B).