This is a 12-month, prospective, multicentre, case-control, observational study comparing effectiveness and safety of COVID-19 vaccine in patients with IBD treated with immunomodulatory drugs (cases) compared with those not treated with the aforementioned drugs (controls). The study should be regarded as observational because the participants will receive the intervention (Covid-19 Vaccine) as part of routine medical care (the administration will be performed outside the study and according to the times and modalities established by the Italian National Vaccination for the whole Italian population aged 18 and over). Then, we aim to assess the effect of this intervention in two predefined groups of individuals (cases and controls). All consecutive patients admitted at each IG-IBD centre that agree to participate in this study could be screened for inclusion. In case of patient's refusal to COVID-19 vaccination, this will be recorded in order to estimate the rate of adherence to the vaccination. Similarly, patients willing to vaccinate but unable to participate in the study due to logistic problems (for example; long distance from the IBD centre) will be recorded.

After enrollment, each patient will be visited at baseline; the interval between basal visit and the subsequent vaccine shall not exceed 10 days. The vaccine will be administered to the patients according to the times and modalities established by the Italian National Vaccination Plan. After the last dose of the vaccine, each patient will enter a 12 month follow-up period. An additional follow-up period will be admitted, as required by clinicians, in order to further evaluate ongoing adverse events.

At baseline, all enrolled patients will undergo a detailed assessment including: demographics, smoking status, body mass index, co-morbidities (presence of diabetes, arterial hypertension, chronic obstructive pulmonary disease, heart disease, and so on), stratification of the disease according to the Montreal classification, activity of the disease (assessed with Harvey-Bradshaw Index for CD, and Partial Mayo Score for UC), current medications, laboratory parameters (haemoglobin, white blood cells, C-reactive protein, faecal calprotectin). Furthermore, all enrolled patients will be tested for the quantitative detection of IgM and IgG antibodies against SARS-CoV-2. In case of IgM positivity, patients will be invited to perform a nasopharyngeal swab for the detection of SARS-CoV-2 RNA. The safety of the vaccine will be assessed by considering the incidence of reported adverse events, which will be reported by the patients during the 10 days following the administration of the vaccine (single assessment in case of single-dose vaccine, double assessment in case of double-dose vaccines, i.e. one evaluation for each dose). After 2 and 12 months from the last dose of the vaccination, the quantitative detection of IgM/IgG antibodies against SARS-CoV-2 will be repeated. The dosage of antibodies will be centrally performed at IEO in Milan. In addition, a proactive surveillance will be performed at 3,6, and 12 months - preferably combined with pre-scheduled direct visits, or alternatively via remote contacts - to detect new or suspected diagnosis of COVID-19. In case of active symptoms suspected for COVID-19 without an established diagnosis, patients will be invited to perform a nasopharyngeal swab for the detection of SARS-CoV-2 RNA. The proactive surveillance will also involve the occurrence of adverse events to the vaccine and the assessment of disease activity to detect flare-ups of IBD (see timeline).

All data will be collected anonymously in a specifically arranged eCRF form inside the IG-IBD registry.