Predicting the protective humoral response to a SARS-CoV-2 mRNA vaccine

Objectives: Simple and standardized methods to establish correlates to vaccine-elicited SARS-CoV-2 protection are needed. Methods: An observational study on antibody response to a mRNA vaccine (Comirnaty) was performed on health care workers (V, n=120). Recovered COVID-19 patients (N, n=94) were used for comparison. Antibody response was evaluated by a quantitative anti-receptor binding domain IgG (anti-RBD) commercial assay and by virus micro neutralization test (MNT), in order to establish a threshold of anti-RBD binding antibody units (BAU) able to predict a robust (>= 1:80) MNT titer. Results: Significant correlation between BAU and MNT titers was found in both V and N, being stronger in V (rs=0.91 and 0.57 respectively, p<0.001); a higher incremental trend starting from MNT titer 1:80 was observed in the V group. The 99% probability of high MNT titer (>= 1:80) was reached at 1,814 and 3,564 BAU/mL, and the area under the receiver operating characteristic (ROC) curve was 0.99 (CI: 0.99-1.00) and 0.78 (CI: 0.67-0.86) in V and N, respectively. Conclusions: A threshold of 2,000 BAU/mL is highly predictive of strong MNT response in vaccinated individuals and may represent a good surrogate marker of protective response. It remains to be established whether the present results can be extended to BAU titers obtained with other assays.