Prolonged steroid treatment has a suppressive effect on the immune system, however, its effect on the cellular response to mRNA vaccine is unknown. Here we assessed the impact of prolonged steroid treatment on the T-cell and humoral response to the SARS-CoV-2 spike (S) peptide following the third dose of the BNT162b2 vaccine in systemic autoimmune rheumatic disease patients. We found that CD4 T-cell response to the S peptide in patients on high-dose long-term steroid treatment showed significantly less S-peptide specific response, compare to low-dose or untreated patients. Remarkably, these results were not reflected in their humoral response, since almost all patients in the cohort had sufficient antibody levels. Moreover, S-peptide activation failed to induce significant mRNA levels of IFNγ and TNFα in patients receiving high-dose steroids. RNA-sequencing datasets analysis implies that steroid treatments' inhibitory effect of nuclear factor kappa-B signaling may interfere with the activation of S-specific CD4 T-cells. This reveals that high-dose steroid treatment inhibits T-cell response to the mRNA vaccine, despite having sufficient antibody levels. Since T-cell immunity is a crucial factor in the immune response to viruses, our findings highlight the need for enhancing the efficiency of vaccines in immune-suppressive patients, by modulation of the T-cell response.