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Objectives To investigate the effects of passive immunization with the anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) monoclonal antibodies tixagevimab/cilgavimab on humoral responses and on coronavirus disease 2019 (COVID-19) outcomes in vaccine-refractory patients with immune-mediated inflammatory diseases (IMIDs) at high risk of severe COVID-19. Methods A prospective cohort study was performed on a cohort of high-risk vaccine-refractory IMID patients treated with a single dose of tixagevimab/cilgavimab (150 mg/150 mg). COVID-19 outcomes as well as serum and salivary anti-SARS-CoV-2 IgG were assessed at baseline and for at least 6 months. Results were compared with an untreated high-risk vaccine-refractory IMID population. Standardized incidence ratios (SIRs) of COVID-19 compared with the general population were calculated for both groups. Results A total of 38 high-risk IMID patients received tixagevimab/cilgavimab and were compared with 114 untreated high-risk IMID controls. Serum anti-spike IgG increased to 6.6 OD (s. d. 0.8) at day 1 and remained positive up to month 6 [6.3 OD (s. d. 1.4)]. Salivary anti-spike IgG peaked at month 2 [1.6 OD (s. d. 1.1)] and decreased from month 3 [0.8 OD (s. d. 0.3)]. No severe or extended infection was observed in the tixagevimab/cilgavimab group. Compared with the general population, the SIR of COVID-19 in treated patients was 0.76 (95% CI 0.24, 1.58) despite the increased risk profile. The SIR of the control group was 1.51 (95% CI 1.07, 2.02), corresponding to a significantly increased incidence. Conclusions Passive immunization with tixagevimab/cilgavimab is safe and effective in inducing anti-SARS-CoV-2 immunity and potentially in preventing COVID-19 in high-risk vaccine-refractory IMID patients. These data provide a proof of concept for the use of monoclonal antibodies as a preventative strategy against SARS-CoV-2 in vulnerable populations. [ABSTRACT FROM AUTHOR]