An expanded myeloid cell compartment is a hallmark of severe coronavirus disease 2019 (COVID-19). However, data regarding myeloid cell expansion have been collected in Europe, where the mortality rate by COVID-19 is greater than those in other regions including Japan . Thus, characteristics of COVID-19-induced myeloid cell subsets remain largely unknown in the regions with low mortality rates . Here, we analyzed cellular dynamics of myeloid-derived suppressor cell (MDSC) subsets and examined whether any of them correlate with disease severity and prognosis, using blood samples from Japanese COVID-19 patients . We observed that polymorphonuclear (PMN) -MDSCs, but not other MDSC subsets, transiently expanded in severe cases but not in mild or moderate cases . Contrary to previous studies in Europe, this subset selectively expanded in survivors of severe cases and subsided before discharge, but such transient expansion was not observed in non-survivors in Japanese cohort . Analysis of plasma cytokine/chemokine levels revealed positive correlation of PMN-MDSC frequencies with interleukin 8 (IL-8) levels prior to the cell expansion, indicating the involvement of IL-8 on recruitment of PMN-MDSCs to peripheral blood following the onset of severe COVID-19 . Thus, our data indicates that transient expansion of the PMN-MDSC subset results in improved clinical outcome . Thus, this myeloid cell subset may be a predictor of prognosis in cases of severe COVID-19 in Japan.