Immune system changes during COVID-19 recovery play key role in determining disease severity.

Coronavirus disease 2019 (COVID-19), an acute respiratory infection, is largely associated with dysregulation and impairment of the immune system. This study investigated how the immune system changes were related to disease severity in COVID-19 patients. The frequencies of different immune cells and levels of pro- and anti-inflammatory cytokines in whole blood of participants were determined by flow cytometry and enzyme-linked immunosorbent assay, respectively. The values of other inflammatory agents were also studied. In the late recovery stage, unlike CD56 CD16 NK cells and monocytes, CD56 CD16 NK cell numbers were increased ( < 0.0001-0.05). Th1, Th2, and Th17 cell percentages were significantly lower in patients than healthy control ( < 0.0001-0.05), while their frequencies were increased following disease recovery ( < 0.0001-0.05). The numbers of Tregs, activated CD4+ T cells, and exhausted CD8+ T cells were significantly decreased during a recovery ( < 0.0001-0.05). No significant change was observed in exhausted CD4+ T cell number during a recovery ( > 0.05). B cell showed an increased percentage in patients compared to healthy subjects ( < 0.0001-0.05), whereas its number was reduced following recovery ( < 0.0001-0.05). IL-1α, IL-1β, IL-6, TNF-α, and IL-10 levels were significantly decreased in the late recovery stage ( < 0.0001-0.05). However, TGF-β1 level was not significantly changed during the recovery ( > 0.05). Lymphocyte numbers in patients were significantly decreased ( < 0.001), unlike ESR value ( < 0.001). Lymphocyte number was negatively correlated to ESR value and Th2 number ( < 0.05), while its association with monocyte was significantly positive at the first day of recovery ( < 0.05). The immune system changes during the disease recovery to improve and regulate immune responses and thereby may associate with the reduction in disease severity.