2.1. Study design

We performed a substudy within an ongoing national prospective observational multicenter cohort study in patients with prevalent IMIDs with the primary objective to study humoral immune responses after vaccination. The Target-to-B! (T2B!) study was approved by the medical ethical committee (NL74974.018.20 and EudraCT 2021-001102-30) and registered in Dutch Trial Register (Trial ID NL8900). All participants provided signed informed consent. Here we report on a predefined secondary outcome of this cohort study, i.e. disease activity of the IMID after vaccination against SARS-CoV-2. Results on humoral responses after vaccinations, incidence of breakthrough infections, and risk factors for short-term adverse events after vaccination against SARS-CoV-2 have been published before [3,12,13]. A detailed description of the T2B! study protocol, including eligibility criteria, was published earlier [3].