Antibody response monitoring after COVID-19 vaccine, comparison of elecsys anti-SARS-COV-2 s test and neutralization test

Objective: Neutralization tests (NT) are the gold standard for measuring anti-SARS-CoV-2 neutralizing antibodies (NAb), but their practical use is limited. Antibodies against the spike (S) protein receptor binding domain (RBD) have been shown to provide a neutralizing activity against SARS-CoV-2. Tests that quantitatively measure anti-S1-RBD-SARS-CoV-2 (Anti-RBD) antibodies could be use for, NAb screening of potential donors for convalescent phase plasma therapy, assessment of innate or vaccine-induced immunity, classification of individuals for vaccination, and vaccine documentation. In our study, it was aimed to compare the two tests by measuring the antibody levels against NAb and S protein RBD with the NT and Elecsys Anti‑SARS‑CoV‑2 S (Elecsys-S) test in the six-month post-vaccination period in patients with COVID-19 vaccine. Materials and Methods: In Ankara City Hospital, blood samples of 200 volunteers vaccinated against COVID-19 was taken at 1, 3, 6 months before and after vaccination; Anti-RBD antibody levels were determined by both NAb and Elecsys-S assay. NT was performed in a BSL-3 laboratory using a Slovenian SARS-CoV-2 isolate; NT titer ≥1:6 was considered positive. The Elecsys-S test was performed on the Cobas e8000 analyzer. The concentration of each sample in U/mL was automatically calculated by the analyzer. Antibodies were measured according to the Cut-Off Index (COI). A COI ≥ 0.8 U/ml was considered positive and a COI < 0.8 U/ml was considered negative Results: The sensitivity of the Elecsys-S test at the first month follow-up was 99.3% and the positive predictive value was 90.8%. At the third and sixth months, the sensitivity of the Elecsys-S test was 100%, and the positive predictive value was 87.5% and 98.3%, respectively. Specificity and negative predictive value could not be calculated since there were no antibody negative patients at the third and sixth months. There was no statistically significant difference in terms of mean age, gender, BMI, smoking, comorbidity, and drug use between neutralizing antibody positive and negative groups. At the third month, there was a moderately positive statistically significant difference between the Elecsys-S test and NT (R: 0.71, p<0.001). In the evaluation of first and 3rd months serum samples (n:274), a weak significant positive correlation was found between Elecsys-S test and NT (R:0.46, p<0.001). A weak statistically significant relationship was found between the Elecsys-S test and NT at 6 months (R: 0.28, p:0.002). Conclusion: When the Elecsys-S test was compared with the NT, which is accepted as the gold standard test, the sensitivity of the Elecsys-S test was found to be very high in the first, third, and sixth month after COVID-19 vaccine. It would be useful to study serum samples with further dilution in a population with high COVID-19 seroprevalence. The Elecsys-S test is a very convenient test to show exposure to COVID-19 with its quantitative results and practical use, but higher threshold values can be used to show protective antibody levels. Key Words: COVID-19, SARS-CoV-2, serological tests, Elecsys Anti-SARS-CoV-2 S test, neutralization test, antibody response