Abstract Objectives To assess immunogenicity of a heterologous fourth dose of an mRNA (BNT162b2) severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine in autoimmune rheumatic diseases (ARD) patients with poor/non-response to inactivated vaccine (Sinovac-CoronaVac). Methods A total of 164 ARD patients who were coronavirus disease 2019 (COVID-19) poor/non-responders (negative anti-SARS-CoV-2 S1/S2 IgG and/or neutralizing antibodies—NAb) to the third dose of Sinovac-CoronaVac received an additional heterologous dose of mRNA (BNT162b2) 3 months after last dose. IgG and NAb were evaluated before and after the fourth dose. Results Significant increases were observed after the fourth dose in IgG (66.4 vs 95.1%, P < 0.001), NAb positivity (5.5 vs 83.5%, P < 0.001) and geometric mean titre (29.5 vs 215.8 AU/ml, P < 0.001), and 28 (17.1%) remained poor/non-responders. Patients with negative IgG after a fourth dose were more frequently under rituximab (P = 0.001). Negative NAb was associated with older age (P = 0.015), RA (P = 0.002), SSc (P = 0.026), LEF (P = 0.016) and rituximab use (P = 0.007). In multiple logistic regression analysis, prednisone dose ≥7.5 mg/day (OR = 0.34; P = 0.047), LEF (OR = 0.32, P = 0.036) and rituximab use (OR = 0.19, P = 0.022) were independently associated with negative NAb after the fourth vaccine dose. Conclusions This is the largest study to provide evidence of a remarkable humoral response after the fourth dose of heterologous mRNA SARS-CoV-2 vaccination in ARD patients with poor/non-response to the third dose of an inactivated vaccine. We further identified that treatment, particularly rituximab and prednisone, impaired antibody response to this additional dose. Trial registration ClinicalTrials.gov, https://clinicaltrials.gov, CoronavRheum #NCT04754698.