Comparison of Anti–SARS-CoV-2 S1 Receptor-Binding Domain Antibody Immunoassays in Health Care Workers Before and After the BNT162b2 mRNA Vaccine

Mariarosa Carta, MD; Irene Marinello, MLS; Anna Cappelletti, MD; Alessandra Rodolfi, MLS; Erica Cerrito, MLS; Camilla Bernasconi, MLS; Marlene Gottardo, MLS; Francesca Dal Lago, MLS; Daniele Rizzetto, MLS; Elena Barzon, MD; Davide Giavarina, MD


Am J Clin Pathol. 2022;157(2):212-218. 

In This Article

Abstract and Introduction


Objectives: The Pfizer-BioNTech BNT162b2 vaccine against SARS-CoV-2 infection is now available. This vaccine induces antibody production against the receptor-binding domain (RBD) of the spike glycoprotein S1 (S1-RBD). This study evaluated the performance of new immunoassays to measure this type of antibody.

Methods: Blood samples were collected at t0 (prime dose), after 21 days (t1, booster dose), and then after another 15 days (t2) from 70 health care professionals who had tested negative for previous SARS-CoV-2 infection and underwent vaccination with BNT162b2.

Results: Antibodies against S1-RBD were measured using 4 commercial assays. At t0, t1, and t2, the median antibody concentrations (interquartile range) were, respectively, 0.2 (0.1–0.4), 49.5 (19.1–95.7), and 888.0 (603.6–1,345.8) U/mL by Maglumi SARS-CoV-2 S-RBD immunoglobulin G (IgG) (Shenzen New Industries Biomedical Engineering, Snibe Diagnostics); 0.0 (0.0–0.0), 7.9 (4.2–15.6), and 112.3 (76.4–205.6) U/mL by Atellica IM SARS-CoV-2 IgG assay (Siemens Healthineers); 0.0 (0.0–0.0), 59.9 (18.3–122.0), and 2,646.0 (1,351.2–4,124.0) U/mL by Elecsys Anti–SARS-CoV-2 S assay (Roche Diagnostics); and 1.8 (1.8–1.8), 184 (94–294), and 1,841.0 (1,080.0–2,900.0) AU/mL by LIAISON SARS-CoV-2 TrimericS IgG assay (DiaSorin). The differences between medians at t0, t1, and t2 were all statistically significant (P < .001).

Conclusions: Antibodies against nucleocapsid proteins (N) were also measured using Maglumi 2019-nCoV IgG assay, which showed all negative results. All the considered anti-RBD methods detected response to the vaccine, while the method directed against anti-N failed to show response.


Since 2000, the COVID-19 pandemic has been conquering the world, leading to millions of people affected. Against this backdrop, vaccination seems to be the best strategy to eradicate the disease. The recent availability of SARS-CoV-2 vaccines could determine a new role for serologic tests. In this context, baseline assessment and postvaccine monitoring of anti–SARS-CoV-2 antibodies may prove instrumental for vaccination strategies (prioritization of individuals with no previous infection) as well as monitoring the extent and duration of the humoral immune response.[1–4]

To achieve this goal, it would be necessary to quantify the antibody concentration, given that interindividual response to a vaccine may differ widely (particularly in patients with previous infection or those on immunosuppressive therapies); in addition, the antibody concentration tends to diminish over time.[4,5]

Serologic assays for SARS-CoV-2 are becoming widely available and include enzyme-linked immunosorbent assay (ELISA), lateral flow assays, virus neutralization assays, and chemiluminescent immunoassays (CLIAs) run on fully automated clinical laboratory instruments.[6] Given the potentially high volume of test requests, the use of fully automated clinical laboratory instruments is advisable.

The immunoassays can detect antibodies with different antigenic targets, produced during SARS-CoV-2 infection, and are mainly directed against nucleocapsid protein (N) or viral spike glycoprotein (S).[7] The first proposed immunoassays targeted SARS-CoV-2 N or S. Most vaccines, however, are currently under emergency use authorization (EUA) in the United States, Europe, and Asia (BNT162b2 [Pfizer-BioNTech], mRNA-1273 [Moderna], Ad26.COV2.S [Janssen/Johnson & Johnson], ChAdOx1 nCoV-19 [AstraZeneca-Oxford University], NVX-CoV2373 [Novavax], and Gam-COVID-Vac [Sputnik V])[8] aim to induce the antibody response exclusively against the spike glycoprotein S1[9–11] and the receptor-binding domain (RBD) of the S1 subunit of the protein spike. These antibodies appear to better correlate with virus neutralization.[12]

At this stage, neither the antibody response nor its magnitude to vaccination, including expected antibody concentrations and measurement differences between different immunoassays, is known. New immunoassays that target the RBD were recently made available.

We chose to evaluate the performance of 4 tests, targeted at RBD antibodies, by comparing them with 1 used in our laboratory, which is targeted at N antibodies. Participants were health care professionals who tested negative to previous SARS-CoV-2 infection through a real-time polymerase chain reaction (PCR) method before and after the BNT162b2 vaccine.