ObjectivesTo evaluate the potential of immunosuppressed patients to mount B-cell and T-cell responses to COVID-19 booster vaccination (third vaccination).MethodsPatients with primary immunodeficiency (PID), immune-mediated inflammatory diseases (IMIDs) on CD20-depleting treatment with rituximab (RTX), or IMIDs treated with conventional synthetic disease-modifying antirheumatic drugs (csDMARDs) or biological disease-modifying antirheumatic drug (bDMARDs) were included and assessed before (baseline visit (BL)) and 2, 4 and 8 weeks after COVID-19 booster vaccination. Serum B-cell responses were assessed by antibody levels against SARS-CoV-2 spike protein (anti-spike IgG antibody (S-AB)) and a surrogate virus neutralisation test (sVNT). T-cell responses were assessed by an interferon gamma release assay (IGRA).ResultsFifty patients with PID (n=6), treated with RTX therapy (n=13), or treated with csDMARDs/bDMARDs (n=31) were included. At BL, anti-S-AB titres in PID and csDMARD/bDMARD-treated patients were low (although significantly higher than RTX patients); measures of B-cell-mediated response increased significantly after booster vaccination. In the RTX cohort, low BL anti-S-AB and sVNT values did not improve after booster vaccination, but patients had significantly elevated IGRA responses post booster vaccination compared with the other groups. csDMARD/bDMARD-treated patients showed the highest BL values in all three assays with greater increases in all parameters after booster vaccination compared with patients with PID.ConclusionPatients with IMID on therapeutic B-cell depletion have low anti-S-AB and sVNT values before and after booster vaccination but show significantly higher levels of IGRA compared with other immunosuppressed patients, suggesting an underlying mechanism attempting to compensate compromised humoral immunity by upregulating T-cell responsiveness. PID appears to have a stronger impact on antiviral immune response than csDMARD/bDMARD treatment.