Background and objectives: People with multiple sclerosis (MS) have an increased risk of severe coronavirus infection due to their level of motor disability or exposure to certain immunosuppressive treatments. Thus, patients with MS have had priority access to coronavirus disease 2019 (COVID-19) vaccination. However, relapses after vaccination have been reported, leading some patients to not seek the recommended booster doses. The main objective was to estimate the risk of severe relapse after 1, 2, and 3 (booster) doses of COVID-19 vaccination in patients with MS. The secondary objectives were to assess the risk of relapse in clinically meaningful subgroups according to the type of vaccine, the characteristics of the patients, and the use of disease-modifying treatments (DMTs). Methods: We conducted a nationwide study using data from the French National Health Data System. Patients with MS were identified according to ICD codes, specific treatments, and reimbursement data up to March 31, 2022. Relapses requiring treatment with high-dose corticosteroids were identified. A self-controlled case series method was used to evaluate the risk of relapse associated with COVID-19 vaccines in the 45 days after vaccination. The associated risk was evaluated after 1, 2, or 3 (booster) doses and is expressed as overall incidence rate ratios (IRRs) and in subgroups of interest. Results: Overall, 124,545 patients with MS were identified on January 1, 2021, and 82% received at least 1 dose of a COVID-19 vaccine (n = 102,524) until December 31, 2021, for a total of 259,880 doses. The combined IRR for MS relapse was 0.97 (0.91-1.03, p = 0.30). The same absence of risk was confirmed in various subgroups (age younger than 50 years, duration of MS < 10 years, use of DMT). A small increase in the relapse risk cannot be excluded after a booster dose (IRR 1.39 [1.08-1.80]) for patients with high MS activity, especially when not treated. Discussion: There is no increased risk of relapse requiring corticosteroid therapy after COVID-19 vaccination for almost all patients. We cannot exclude an increased risk after the booster dose for patients who have had at least 2 relapses in the previous 2 years. Classification of evidence: This study provides Class III evidence that COVID-19 vaccination does not increase the risk of severe relapse in patients with MS.