Abstract: The present disclosure an ELISA-based assay that uses a glycosylated s1F polypeptide fragment derived from the SARS-CoV-2 spike protein (Covid-19) spike protein, the N-terminal domain of which has affinity for the tyrosine-protein kinase receptor UFO (AXL). The S1F polypeptide can be generated by expression of an encoding nucleic acid by a human cell expression system resulting in glycosylation of the expressed S1F polypeptide at least at the N343 N-glycosylation site thereof, and which surprisingly and significantly increases the affinity of the S1F for AXL, provides a significant increase in the sensitivity of the assay compared to other known assays. Further the AXL polypeptide can be glycosylated, which further increases the affinity for S1F and AXL to each other.