The Comparability of Anti-SARS-CoV-2 IgG Levels Measured with Different Immunoassays Varies over Time after Primary BNT162b2 Vaccination and Homologous Booster Immunization

This serosurveillance study included 74 health care workers (43±12 years, 41 females) of Pederzoli Hospital, who voluntarily received primary vaccination with Pfizer/BioNTech BNT162b2 (double 30-μg dose with 3-weeks interval), followed by homologous booster (30-μg). Serum was used for testing anti-SARS-CoV-2 IgG antibodies with DiaSorin Trimeric spike IgG immunoassay and Maglumi SARS-CoV-2 S-RBD IgG. Spearman’s correlation between the two immunoassays remained always considerably high, ranging between 0.81 and 0.89, reaching the highest value 1 month after booster immunization. The values of DiaSorin Trimeric spike IgG were consistently higher than those obtained with Maglumi SARS-CoV-2 S-RBD IgG, and the ratio between these tests gradually increased over time from the administration of the first vaccine dose up to 9 months after the second dose (i.e., the ratio increased from 2.5 to 10.5). A different trend was then noted 1 month after booster immunization, with the bias between these tests decreasing to 6.3. These results confirm that the comparability of anti-Spike SARS-CoV-2 antibody tests, even within the same IgG antibody class is always high, but the bias of varies widely depending on timing of sample collection after primary and even after booster immunization.