BackgroundThe decline of humoral response to COVID-19 vaccine led to authorise a booster dose. Here, we characterised the kinetics of B-cell and T-cell immune responses in patients with multiple sclerosis (PwMS) after the booster dose.MethodsWe enrolled 22 PwMS and 40 healthcare workers (HCWs) after 4–6 weeks from the booster dose (T3). Thirty HCWs and 19 PwMS were also recruited 6 months (T2) after the first dose. Antibody response was measured by anti-receptor-binding domain (RBD)-IgG detection, cell-mediated response by an interferon (IFN)-γ release assay (IGRA), Th1 cytokines and T-cell memory profile by flow cytometry.ResultsBooster dose increased anti-RBD-IgG titers in fingolimod-treated, cladribine-treated and IFN-β-treated patients, but not in ocrelizumab-treated patients, although antibody titres were lower than HCWs. A higher number of fingolimod-treated patients seroconverted at T3. Differently, T-cell response evaluated by IGRA remained stable in PwMS independently of therapy. Spike-specific Th1-cytokine response was mainly CD4⁺T-cell-mediated, and in PwMS was significantly reduced (p<0.0001) with impaired IL-2 production compared with HCWs at T3. In PwMS, total Th1 and IFN-γ CD4⁺T-cell responders to spike protein were increased from T2 to T3.Compared with HCWs, PwMS presented a higher frequency of CD4⁺and CD8⁺terminally differentiated effector memory cells and of CD4⁺effector memory (T_EM) cells, independently of the stimulus suggesting the association of this phenotype with MS status. CD4⁺and CD8⁺T_EMcell frequency was further increased at T3 compared with T2.ConclusionsCOVID-19 vaccine booster strengthens humoral and Th1-cell responses and increases T_EMcells in PwMS.