Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in late 2019, causing the coronavirus disease 2019 (COVID-19) pandemic with profound global impacts, prompting urgent vaccine development that achieved high efficacy against initial strains. Nevertheless, the virus's antigenic evolution led to variants capable of immune evasion, particularly Omicron in 2021 and its sublineages, which fueled widespread infections through numerous spike protein mutations that impair neutralizing antibody binding. Elucidating the long-term durability of immunity against Omicron is crucial for advancing understanding of sustained defense and refining public health vaccination strategies.

COVID-19 vaccines elicit adaptive immune responses, including humoral immunity via binding and neutralizing antibodies and cell-mediated immunity through memory T and B cells. In this study, we longitudinally monitored the immune responses of vaccinated Finnish healthcare workers over more than three years. We used enzyme immunoassay (EIA) to analyze binding immunoglobulin G (IgG) antibodies, a microneutralization test (MNT) to quantify neutralizing antibodies against Omicron variants, and an activation-induced marker (AIM) assay with flow cytometry to measure CD4⁺ and CD8⁺ T cell activation. In addition, we developed an ELISpot assay to quantify the number of memory B cells.

The results indicate differences between vaccine types and regimens. However, in all cases, antibody levels gradually waned after each dose but were effectively restored by following boosters, though Omicron variants exhibited partial escape from neutralization. Hybrid immunity sustained elevated antibody levels and broader cross-neutralization against variants. T cells were durable and cross-reactive against Omicron, memory T cell subsets persisted, and functional memory B cells were maintained. These findings underscore the persistence of T cells and hybrid immunity against Omicron variants, simultaneously emphasizing the value of booster vaccinations in maintaining protection.