This process is driven by high affinity between RBD and its cellular receptor. The efficiency of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections is dependent on the efficiency of binding between the receptor-binding domain (RBD) of the viral spike protein S1 and the cellular angiotensin-converting enzyme 2 (ACE2) receptor. Therefore, parallel avidity testing of RBD WT and omicron seems to be mandatory for a significant assessment of protective immunity towards SARS-CoV-2. In contrast, the avidity of IgG cross-reacting with RBD of the omicron variant was always much lower than for IgG RBD WT, except after the third vaccination step. After infection with WT SARS-CoV-2 or vaccination based on mRNA WT, the avidity of cross-reacting IgG directed towards RBD of the delta variant only showed marginal differences compared to IgG directed towards RBD WT. High avidity is not reached by infection alone. Our data confirm that natural SARS-CoV-2 infection or one vaccination step lead to low avidity IgG, whereas further vaccination steps gradually increase avidity to high values. Using a prototype line assay based on the established recomLine SARS-CoV-2 assay, supplemented with RBD of the delta and the omicron variant, differential avidity determination of IgG directed towards RBD of wild-type (WT) SARS-CoV-2 and distinct variants was possible within one assay. It seems to be linked to increased infection-neutralization potential and therefore might indicate protective immunity. The avidity (binding strength) of IgG directed towards the receptor-binding domain (RBD) of spike protein has been recognized as a central marker in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) serology.
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