Longitudinal analysis of anti-SARS-CoV-2 S-RBD IgG antibodies before and after the third dose of the BNT162b2 vaccine

Abstract

Immunosurveillance by evaluating anti-spike protein receptor-binding domain (S-RBD) antibodies represents a useful tool to estimate the long immunity against Severe Acute Respiratory Syndrome CoronaVirus 2 (SARS-CoV-2) infection. The aim of this study was to evaluate the kinetics of antibody response in vaccine recipients. We measured anti-S-RBD IgG levels by indirect chemiluminescence immunoassay on Maglumi 800 (SNIBE, California) in 1013 healthy individuals naïve to SARS-CoV2 infection after two and three COVID-19 vaccine doses. We found that anti-S-RBD IgG levels are higher in females than males. Antibody levels gradually decrease to a steady state after four months since the peak, and the decay is independent of age, sex, vaccine doses, and baseline antibodies titer. The third dose induces a high anti-S-RBD IgG reactivity in individuals with previous high responses and triggers a moderate-high anti-S-RBD IgG reactivity. The assessment of anti-S-RBD IgG levels is essential for monitoring long-term antibody response. A third SARS-CoV-2 vaccine dose is associated with a significant immunological response. Thus, our results support the efficacy of the vaccine programs and the usefulness of the third dose.

Figure 1

Line plot for the % residual IgG anti-S-RBD reactivity. Each point represents a median. Individuals were sub-grouped according to their baseline anti-S-RBD IgG reactivity.

Figure 2

Line plot for the IgG anti-S-RBD reactivity kinetics before and after the booster dose. Days were counted from the first measurement (about 1 month after immunization). For each line, the point at the beginning of the last ascending segment represents the day of the booster dose. The colour of the line represents anti-S-RBD IgG levels reached after immunization (black: > 2000 BAU/mL, red: 1000–2000 BAU/mL, blue: 500–1000 BAU/mL, orange: < 500 BAU/mL). IgG levels are truncated at 4330 BAU/mL, the maximum measurable level (samples with higher titers were not diluted).