To discover immune factors that can predict the progression of COVID-19, we evaluated circulating immune cells and plasma cytokines in COVID-19 patients . We found that T cells, including CD4+ T cells and CD8+ T cells, were significantly decreased in severe COVID-19 symptoms but not in mild symptoms, in comparison with healthy people . T cells remained at a low level after recovery from severe COVID-19 . CD4+CD25+CD127low Treg-enriched cells were significantly increased in either mild or severe COVID-19 patients, regardless of recovery or not . Moreover, in either mild or severe COVID-19 patients, Treg-enriched cells up-regulated CD25 and down-regulated CD127 . After recovery, CD25 was partially down-regulated but still higher than the normal level, while CD127 returned to the normal level in mild patients but not severe patients . B cells were decreased in mild patients and further decreased in severe patients, and remained low after recovery . NK cells were decreased only in severe COVID-19, with a tendency to return to the normal level after recovery . Plasma IL-6 and IL-10 were both elevated in severe patients but not in mild patients . After recovery, IL-6 remained higher than its normal level, while IL-10 returned to the normal level . Binary logistic regression analysis indicated that CD4+ T cells, B cells, IL-6, and IL-10 were significantly associated with COVID-19 severity . Therefore, these parameters are indicators of COVID-19 severity . Dynamic monitoring of these parameters would benefit therapy planning and prognosis evaluation.