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Background: Alongside vaccine hesitancy, impaired and waning immunity in autoimmune rheumatic diseases (ARDs) are barriers to immunization. The timeframe of immunity waning in ARD remains unclear., Objective: We aimed to examine the waning of humoral immunogenicity in a cohort of ARD patients who received the heterologous inactivated vaccine followed by the adenoviral vector SAR-CoV-2 vaccine at a 3-month follow-up., Methods: The levels of SARS-CoV-2 anti-RBD IgG were evaluated at 1 and 3 months in adults with ARDs (n = 29) and age- and sex-matched healthy controls (HC) that received the heterologous prime-boost CoronaVac vaccine followed by the ChAdOx1 nCoV-19 vaccine. Seropositivity was defined as anti-receptor binding domain (RBD) IgG levels of ≥ 7.15 binding antibody units (BAU)/mL. The kinetic properties of the vaccines were evaluated based on the ratio of anti-RBD IgG values obtained at each follow-up. Disease activity was evaluated., Results: The seropositivity rate was lower among patients with ARDs than among HCs (89.7% vs. 100%, p = 0.237). At 3 months, the median (IQR) anti-RBD IgG level was lower among patients with ARDs than among HCs (122.3 [30.6, 247.8] vs. 294.2 [127.4,605.7] BAU/mL, p = 0.006). Mean antibody levels in patients with ARDs decreased 3.5 (1.9)-fold within 3 months post-vaccination (122.3 [30.6, 247.8] vs. 279.9 [86,1076.5] BAU/mL, p < 0.001). Disease flare-ups occurred in three patients., Conclusions: Our findings included changes to anti-RBD IgG levels and may inform vaccination strategies. SAR-CoV2 vaccine-induced immunity was lower in patients with ARDs than in HCs and decreased within 3 months, suggesting a need for booster vaccinations.