Introduction: Several studies reported on the humoral response in subjects having received the
BNT162b2 mRNA COVID-19 vaccine. However, data on the kinetics of antibodies 3 months postvaccination
are currently lacking and are important to drive the future vaccination strategy.
Methods: The CRO-VAX HCP study is an ongoing multicenter, prospective and interventional study
designed to assess the antibody response in a population of healthcare professionals who had received
two doses of the BNT162b2 mRNA COVID-19 vaccine. Two-hundred individuals underwent a blood
drawn within 2 days before the first vaccine dose. One-hundred and forty-two persons (71%) were
categorized as seronegative at baseline while 58 (29%) were seropositive. Samples were then collected
after 14, 28, 42, 56, and 90 days. Antibodies against the SARS-CoV-2 nucleocapsid and the receptor
binding domain of the S1 subunit of the spike protein were measured in all individuals at different time
Results: Using a one-compartment kinetics model, the time to maximum concentration was estimated
at 36 ± 3 days after the first dose and the estimated half-life of antibodies was 55 days (95% CI: 37-107
days) in seronegative participants. In seropositive participants, the time to maximum concentration
was estimated at 24 ± 4 days and the estimated half-life was 80 days (95% CI: 46-303 days). The
antibody response was higher in seropositive compared to seronegative participants.
Conclusion: In both seropositive and seronegative subjects, a significant antibody decline was
observed at 3 months compared to the peak response. Nevertheless, the humoral response remained
robust in all participants.