Real-world data regarding the effectiveness, safety and immunogenicity of the Pfizer-BioNTech BNT162b2 mRNA vaccine are accumulating in the literature, suggesting that this vaccine generates high titres of S1-binding IgG antibodies that exhibit potent virus neutralization capacity. This is the first phase IV immunogenicity study to recruit a large number of Greek healthcare workers (n=425) including 63 previously-infected subjects. We measured titres of neutralizing IgGs against the receptor-binding domain of the S1 subunit of the spike protein of SARS-CoV-2 14 days post-immunization with the first dose, employing the SARS-CoV-2 IgG II Quant assay. A total of 92.24 % of our study cohort received a positive assay outcome and titres varied with age. Post-hoc analysis revealed that although titres did not significantly differ among participants aged 20–49 years, a significant decline was marked in the age group of 50–59 years, which was further accentuated in subjects aged over 60. Antibody titres escalated significantly among the previously-infected, indicating the potential booster effect of the first dose in that group.