We aimed to compare the spectrum and severity of COVID-19 and vaccine breakthrough infections (BIs) among patients with IIMs, other systemic autoimmune and inflammatory diseases (SAIDs), and healthy controls (HCs). This is a cross-sectional study with data from the COVAD study, a self-reported online global survey that collected demographics, COVID-19 history, and vaccination details from April to September 2021. Adult patients with at least one COVID-19 vaccine dose were included. BIs were defined as infections occurring > 2 weeks after any dose of vaccine. Characteristics associated with BI were analyzed with a multivariate regression analysis. Among 10,900 respondents [42 (30-55) years, 74%-females, 45%-Caucasians] HCs were (47%), SAIDs (42%) and IIMs (11%). Patients with IIMs reported fewer COVID-19 cases before vaccination (6.2%-IIM vs 10.5%-SAIDs vs 14.6%-HC; OR = 0.6, 95% CI 0.4-0.8, and OR = 0.3, 95% CI 0.2-0.5, respectively). BIs were uncommon (1.4%-IIM; 1.9%-SAIDs; 3.2%-HC) and occurred in 17 IIM patients, 13 of whom were on immunosuppressants, and 3(18%) required hospitalization. All-cause hospitalization was higher in patients with IIM compared to HCs [23 (30%) vs 59 (8%), OR = 2.5, 95% CI 1.2-5.1 before vaccination, and 3 (18%) vs 9 (5%), OR = 2.6, 95% CI 1.3-5.3 in BI]. In a multivariate regression analysis, age 30-60 years was associated with a lower odds of BI (OR = 0.7, 95% CI 0.5-1.0), while the use of immunosuppressants had a higher odds of BI (OR = 1.6, 95% CI 1.1-2.7). Patients with IIMs reported fewer COVID-19 cases than HCs and other SAIDs, but had higher odds of all-cause hospitalization from COVID-19 than HCs. BIs were associated with the use of immunosuppressants and were uncommon in IIMs. The online version contains supplementary material available at 10.1007/s00296-022-05229-