ObjectiveTo evaluate the effectiveness of COVID-19 vaccination in reducing incident and severe COVID-19 among pregnant individuals with autoimmune rheumatic diseases (AIRDs).MethodsWe conducted a retrospective cohort study using national electronic health records, including 270,811 pregnant individuals aged 15 to 55 years with pregnancies between December 10, 2020, and June 1, 2024. Among them, 4,715 (1.5%) had at least one AIRD (rheumatoid arthritis, spondyloarthritis, or systemic lupus erythematosus). Participants were categorized by AIRD and vaccination status (unvaccinated, initially vaccinated, or booster vaccinated). Time to incident and severe COVID-19 was assessed using adjusted time ratios (TRs), stratified by variant period (pre-Omicron vs Omicron).ResultsInitial vaccination delayed time to COVID-19 during the pre-Omicron period in individuals with AIRDs (TR: 1.095; 95% confidence interval [CI]: 1.036-1.157) but not during Omicron (TR: 1.039; 95% CI: 0.980-1.102). Booster vaccination was not significantly protective against infection (TR: 1.039; 95% CI: 0.958-1.128) or severe outcomes (TR: 1.119; 95% CI: 0.670-1.867). Compared to vaccinated individuals without AIRDs, those with AIRDs had shorter time to breakthrough infections in both pre-Omicron (TR: 0.940; 95% CI: 0.891-0.991) and Omicron (TR: 0.905; 95% CI: 0.858-0.955) periods and to severe disease during Omicron (TR: 0.631; 95% CI: 0.485-0.821).ConclusionVaccination is associated with protection against COVID-19 in pregnancy, but its effectiveness is reduced in those with AIRDs, particularly during the Omicron period.