Assessment of the Humoral Immune Response Following COVID-19 Vaccination in Healthcare Workers: A One Year Longitudinal Study

The continuous variability of SARS-CoV-2 and the rapid waning of specific antibodies threatens the efficacy of COVID-19 vaccines. We aimed to evaluate antibody kinetics one year after SARS-CoV-2 vaccination with an mRNA vaccine in healthcare workers (HCW), with or without a booster. A marked decline in anti-Spike(S)/Receptor Binding Domain (RBD) antibody levels was registered during the first eight months post-vaccination, followed by a transitory increase after the booster. At three months post-booster an increased antibody level was maintained only in HCW vaccinated after a prior infection, who also developed a higher and long-lasting level of anti-S IgA antibodies. Still, IgG anti-nucleocapsid (NCP) fades five months post-SARS-CoV-2 infection. Despite the decline in antibodies one-year post-vaccination, 68.2% of HCW preserved the neutralization capacity against the ancestral variant, with a decrease of only 17.08% in the neutralizing capacity against the Omicron variant. Nevertheless, breakthrough infections were present in 6.65% of all participants, without any correlation with the previous level of anti-S/RBD IgG. Protection against the ancestral and Omicron variants is maintained at least three months after a booster in HCW, possibly reflecting a continuous antigenic stimulation in the professional setting.