Severe COVID-19 is associated with eosinophil disappearance and accumulation of immature neutrophils with impaired nigericin-triggered caspase-1 activation. Whole peripheral blood cells of healthy donors or COVID-19 patients were analyzed by flow cytometry using CD45, CD66b, CD16, and Siglec-8 or CD45, CD66b, CD16, CD10, and CD15 markers. (A) CD66b⁺CD16^dim cells were analyzed for CD45 expression. (B) CD66b⁺CD16^dim cells were analyzed for CD66b and Siglec-8 (eosinophil marker) and the frequency of CD66b⁺CD16^dimSiglec8⁺ eosinophils among leukocytes was determined. (C) CD66b⁺CD16^dim cells were analyzed for CD10 (marker of mature neutrophil) and CD15 (neutrophil marker) and the frequency of CD66b⁺CD16^dimCD15⁺CD10⁻ immature neutrophils among leukocytes was determined. (D-G) Whole peripheral blood was treated with vehicle (control) or nigericin (5 µM) for 30 minutes and eosinophils or immature neutrophils were analyzed for FAM-FLICA MFI (caspase-1 activation) (D,F) and nigericin-induced fold of FAM-FLICA MFI compared with control (E,G). (H) Peripheral blood cells of recovered COVID-19 patients were collected 30 to 50 days after the first analysis. Whole peripheral blood cells of recovered COVID-19 patients were analyzed by flow cytometry using CD45, CD16, and CD66b markers. CD66b⁺CD16^dim cells were analyzed for CD45 expression. *P ≤ .05; **P ≤ .01; ****P < .001. D, diseased; R, recovered.