NEFL and COVID-19: The pooled fixed-effects meta-analysis revealed a significant difference in NfL levels between patients with severe COVID-19 and healthy controls (SMD = 1.13; 95% CI: 0.94, 1.32; Z = 11.82; p ≤ 0.001), although there was no significant heterogeneity (I2: 46%; p > 0.05) (bottom panel, Figure 5).