This ELISA kit against the S Protein has been calibrated against the WHO First International Standard for anti-SARS-CoV-2 immunoglobulin (human), NIBSC Code 20/136. In this way, it is possible to quantitatively report the concentration of IgG immunoglobulins in United Antibody Units (BAU / ml) against protein S. For this, the kit includes a positive control of known concentration with which to make a calibration line through serial dilutions.