In patients with COVID-19, serum concentrations of NfL and GFAP rose in a severity-dependant manner at both the acute and subacute time points, with a magnitude equal to the levels seen following severe traumatic brain injury in some patients; there was no consistent difference between serum total tau concentrations between patients and controls (Fig. 1A and B and Supplementary Table 1). Here, GFAP is linked to COVID-19.