In terms of inflammatory biomarkers, MSCs reduced the levels of C-reactive protein (n = 88; MD =  − 32.49; 95% CI =  − 48.43, − 16.56; P = 0.46; I2 = 0%) and interferon-gamma (n = 44; SMD =  − 1.23; 95% CI =  − 1.89, − 0.57; P = 0.37; I2 = 0%) in severe COVID-19 patients but had no significant effect on interleukin-6 (n = 185; MD =  − 0.75; 95% CI =  − 7.76, 6.27; P = 0.57; I2 = 0%). Here, IL6 is linked to COVID-19.