After administration of a third dose of mRNA vaccine, mean cross-neutralizing antibody responses against the Omicron variant of SARS-CoV-2 were 50.3% in health-care workers but only 26.8% in patients with autoimmune rheumatic diseases (ARDs) in an observational cohort study. Responses capable of neutralizing the Omicron variant were detected in only 39.2% of sera from patients with ARDs who received a third dose of mRNA vaccine. Among the 19 patients with ARDs with confirmed breakthrough infections in the subsequent observation period, 14 (73.7%) had not reached the Omicron-neutralization capacity threshold pre-infection.