Evaluation of commercial anti-SARS-CoV-2 antibody assays and comparison of standardized titers in vaccinated healthcare workers.

Abstract With the availability of vaccines, commercial assays detecting anti-SARS-CoV-2 antibodies (Ab) evolved towards quantitative assays directed to the spike glycoprotein or its receptor binding domain (RBD). The main objective of the present study was to compare the Ab titers obtained with quantitative commercial binding Ab assays, after 1 dose (convalescent individuals) or 2 doses (naive individuals) of vaccine, in healthcare workers (HCW). Antibody titers were measured in 263 sera (from 150 HCW) with 5 quantitative immunoassays (Abbott RBD IgG II quant, bioMerieux RBD IgG, DiaSorin Trimeric spike IgG, Siemens Healthineers RBD IgG, Wantai RBD IgG). One qualitative total antibody anti RBD detection assay (Wantai) was used to detect previous infection before vaccination. The results are presented in binding Ab units (BAU)/mL after application, when possible, of a conversion factor provided by the manufacturers and established from a World Health Organization (WHO) internal standard. There was a 100% seroconversion with all assays evaluated after two doses of vaccine. With assays allowing BAU/ml correction, Ab titers were correlated (ρ= 0.84-0.99). However, a significant difference between values persisted. The titer differences varied by a mean 3.04% between Siemens and bioMerieux assays to 50.54% between Siemens and DiaSorin assays. Titer harmonization is still to be improved despite better results were obtained between assays detecting the same Ab against the same antigen. The next step towards a true standardization of the assays would be to include the International Standard in the calibration of each assays to express the results in IU/mL.