Antibody response to four doses of SARS-CoV-2 vaccine in rare autoimmune rheumatic diseases: an observational study.

Objective: Antibody responses to coronavirus disease 2019 (COVID-19) vaccines are reduced among immunocompromised patients but are not well quantified among people with rare disease. We conducted an observational study to evaluate the antibody responses to the booster SARS-CoV-2 vaccine in people with rare autoimmune rheumatic diseases (RAIRD). Methods: Blood samples were collected after second, before third, after third and after fourth vaccine doses. Anti-spike and anti-nucleocapsid antibody levels were measured using an in-house ELISA. Logistic regression models were built to determine the predictors for non-response. Results were compared with age- and sex-matched healthy controls. Results: Forty-three people with RAIRD were included, with a median age of 56 years. Anti-spike seropositivity increased from 42.9% after second dose to 51.2% after third dose and 65.6% after fourth dose. Median anti-spike antibody levels increased from 33.6 (interquartile range 7.8-724.5) binding antibody units after second dose to 239.4 (interquartile range 35.8-1051.1) binding antibody units after the booster dose (third dose, or fourth dose if eligible). Of the participants who had sufficient antibody levels post-second dose, 22.2% had insufficient levels after the booster, and 34.9% of participants had lower antibodies after the booster than the lowest healthy control had after the second dose. Rituximab in the 6 months prior to booster ( Conclusion: Although the booster dose increased anti-spike IgG and seropositivity rates, some people with RAIRD, particularly those on rituximab, had insufficient antibody levels despite three or four doses.