Background Data are scarce on immunogenicity of COVID-19 vaccines in patients with autoimmune diseases, who are often treated with immunosuppressive drugs. We aimed to investigate the effect of different immunosuppressive drugs on antibody development after COVID-19 vaccination in patients with autoimmune diseases. Methods In this study, we used serum samples collected from patients with autoimmune diseases and healthy controls who were included in two ongoing prospective cohort studies in the Netherlands. Participants were eligible for inclusion in this substudy if they had been vaccinated with any COVID-19 vaccine via the Dutch national vaccine programme, which at the time was prioritising vaccination of older individuals. Samples were collected after the first or second COVID-19 vaccination. No serial samples were collected. Seroconversion rates and IgG antibody titres against the receptor-binding domain of the SARS-CoV-2 spike protein were measured. Logistic and linear regression analyses were used to investigate the association between medication use at the time of vaccination and at least until sampling, seroconversion rates, and IgG antibody titres. The studies from which data were collected are registered on the Netherlands Trial Register, Trial ID NL8513, and ClinicalTrials.org, NCT04498286. Findings Between April 26, 2020, and March 1, 2021, 3682 patients with rheumatic diseases, 546 patients with multiple sclerosis, and 1147 healthy controls were recruited to participate in the two prospective cohort studies. Samples were collected from patients with autoimmune diseases (n=632) and healthy controls (n=289) after their first (507 patients and 239 controls) or second (125 patients and 50 controls) COVID-19 vaccination. The mean age of both patients and controls was 63 years (SD 11), and 423 (67%) of 632 patients with autoimmune diseases and 195 (67%) of 289 controls were female. Among participants without previous SARS-CoV-2 infection, seroconversion after first vaccination were significantly lower in patients than in controls (210 [49%] of 432 patients vs 154 [73%] of 210 controls; adjusted odds ratio 0·33 [95% CI 0·23–0·48]; p<0·0001), mainly due to lower seroconversion in patients treated with methotrexate or anti-CD20 therapies. After the second vaccination, seroconversion exceeded 80% in all patient treatment subgroups, except among those treated with anti-CD20 therapies (three [43%] of seven patients). We observed no difference in seroconversion and IgG antibody titres between patients with a previous SARS-CoV-2 infection who had received a single vaccine dose (72 [96%] of 75 patients, median IgG titre 127 AU/mL [IQR 27–300]) and patients without a previous SARS-CoV-2 infection who had received two vaccine doses (97 [92%] of 106 patients, median IgG titre 49 AU/mL [17–134]). Interpretation Our data suggest that seroconversion after a first COVID-19 vaccination is delay...

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