The RE meta-analysis showed that patients with COVID-19 had significantly higher levels of NfL when compared to the healthy controls (SMD = 0.60; 95% CI: 0.37, 0.82; Z = 5.23; p ≤ 0.001), with significant heterogeneity across studies (I2: 84%; p ≤ 0.001) (top panel, Figure 4). Here, NEFL is linked to COVID-19.