CD4 and respiratory tract infectious disorder: Compared to participants who were recovered from other respiratory infections, there was an expansion of regulatory T-cells (Tregs, defined as CD45+CD3+CD4+CD25+CD127lowFOXP3+) and a decrease in natural killer (NK) cells (CD45+CD56+/NKp46+) in COVID-19 patients at 1–3 months post-symptom onset that returned by 6–9 months post-symptom onset to levels observed in individuals with non-COVID-19 respiratory infections (Figure 4d,g).