Kinetics of Humoral Immunity against SARS-CoV-2 in Healthcare Workers after the Third Dose of BNT162b2 mRNA Vaccine

Protection provided by COVID-19 vaccines is compromised due to waning immunity over time. This study aimed to assess the level of antibodies anti-S-RBD of SARS-CoV-2 in a cohort of healthcare workers before and, on average, one and four months after the third dose of the BNT162b2 vaccine. The determination of antibodies was carried out in serum samples using an electrochemiluminescence immunoassay (ECLIA). All 34 participants (10 males, 24 females, 19 participants <50 years old, 15 participants >= 50 years old) showed a significant antibody level increase after the booster dose. Subsequently, a significant decrease in the antibody concentration was observed, with a reduction of about 60% after 150 days from the booster. Six subjects were infected by SARS-CoV-2 after the booster and showed a significantly higher antibody concentration on average four months after the third dose compared to naive ones. Male and female participants had a similar trend in the antibody decline, while older subjects, compared to the younger ones, had a slightly slower decrease, even if they developed a lower level of antibodies after the third dose. These findings support the importance of the booster dose and underline the need for surveillance programs to better understand the antibody kinetics and optimize vaccination strategies.