The pronounced decline of anti-SARS-CoV-2 spike trimeric IgG and RBD IgG in baseline seronegative individuals six months after BNT162b2 vaccination is consistent with the need for vaccine boosters

Background: This observational retrospective study aimed to define the kinetics of serum levels of anti-SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) spike trimeric and anti-receptor binding domain (RBD) IgG antibodies up to 6 months after BNT162b2 vaccination. Methods: The sample consisted of 86 SARS-CoV-2 baseline seronegative subjects (median age 45 years, IQR 31-53 years; 52.3% females) undergoing vaccination with Pfizer/BioNTech BNT162b2. Blood was drawn before receiving the first and the second vaccine dose, as well as 1, 3 and 6 months after the second vaccine dose. The serum levels of anti-SARS-CoV-2 spike trimeric IgG and RBD IgG antibodies were assayed. Results: The peak of both antibodies types was reached 1 month after the second dose (2808 and 2163 BAU/mL for anti-SARS-CoV-2 spike trimeric IgG and RBD IgG), after which serum levels progressively declined, falling after 6 months to 486 BAU/mL and 167 BAU/mL for anti-SARS-CoV-2 spike trimeric IgG and RBD IgG, respectively. The median rate of 6-month decline was 85% and 93% for anti-SARS-CoV-2 spike trimeric IgG and RBD IgG, respectively. The rate of vaccine recipients with serum antibodies levels above the 80% threshold of vaccine efficacy declined from over 95% at the peak to 72% and 5% at 6 months for anti-SARS-CoV-2 spike trimeric IgG and RBD IgG, respectively. Conclusions: The results of this retrospective observational study are consistent with the need for timely administration of vaccine boosters to prevent that humoral immunity will wane.