AIMS: This study aimed to make a comparison between the clinical laboratory-related factors, complete blood count (CBC) indices, cytokines, and lymphocyte subsets in order to distinguish severe coronavirus disease 2019 (COVID-19) cases from the non-severe ones. MATERIALS AND METHODS: Relevant studies were searched in PubMed, Embase, Scopus, and Web of Science databases until March 31, 2020. Cochrane's Q test and the I(2) statistic were used to determine heterogeneity. We used the random-effect models to pool the weighted mean differences (WMDs) and 95% confidence intervals (CIs). KEY FINDINGS: Out of a total of 8557 initial records, 44 articles (50 studies) with 7865 patients (ranging from 13 to 1582), were included. Our meta-analyses with random-effect models showed a significant decrease in lymphocytes, monocyte, CD4+ T cells, CD8+ T cells, CD3 cells, CD19 cells, and natural killer (NK) cells and an increase in the white blood cell (WBC), neutrophils, neutrophil to lymphocyte ratio (NLR), C-reactive protein (CRP)/hs-CRP, erythrocyte sedimentation rate (ESR), ferritin, procalcitonin (PCT), and serum amyloid A (SAA), interleukin-2 (IL-2), IL-2R, IL-4, IL-6, IL-8, IL-10, tumor necrosis factor-alpha (TNF-alpha), and interferon-gamma (INF-gamma) in the severe group compared to the non-severe group. However, no significant differences were found in IL-1beta, IL-17, and CD4/CD8 T cell ratio between the two groups. SIGNIFICANCE: Decrease in total lymphocytes and lymphocyte subsets as well as the elevation of CRP, ESR, SAA, PCT, ferritin, and cytokines, but not IL-1beta and IL-17, were closely associated with COVID-19 severity, implying reliable indicators of severe COVID-19.