BACKGROUND AND PURPOSE: We evaluated whether there was a difference in the occurrence of relapses pre- and post-COVID-19 vaccination in a nationwide cohort of Danish patients with relapsing multiple sclerosis. METHODS: We conducted a population-based, nationwide cohort study with a cutoff date of 1 October 2022. We used McNemar tests to assess changes in the proportion of patients with recorded relapses within 90 days and 180 days before and after first vaccine dose, and a negative binomial regression model to compare the 90 and 180 days postvaccination annualized relapse rate (ARR) to the 360 days prevaccination ARR. Multivariate Cox regression was used to estimate relapse risk factors. RESULTS: We identified 8169 vaccinated (87.3% Comirnaty) patients without a recorded history of a positive COVID-19 test. We did not find statistically significant changes in the proportion of patients with relapses in the 90 days (1.3% vs. 1.4% of patients, p = 0.627) and 180 days (2.7% vs. 2.6% of patients, p = 0.918) pre- and postvaccination. Also, a comparison of the ARR 360 days before (0.064, 95% confidence interval [CI] = 0.058-0.070) with the ARR 90 (0.057, 95% CI = 0.047-0.069, p = 0.285) and 180 (0.055, 95% CI = 0.048-0.063, p = 0.060) days after vaccination did not show statistically significant differences. Lower age, higher Expanded Disability Status Scale score, and relapse within 360 days before vaccination were associated with a higher risk of relapse. CONCLUSIONS: We did not find evidence of increased relapse activity following the administration of the first dose of the COVID-19 vaccine.

Danish Multiple Sclerosis Centre Department of Neurology Copenhagen University Hospital Rigshospitalet Glostrup Denmark

Danish Multiple Sclerosis Registry Department of Neurology Copenhagen University Hospital Rigshospitalet Glostrup Denmark

Department of Clinical Medicine Faculty of Health and Medical Sciences University of Copenhagen Copenhagen Denmark

Department of Neurology and Centre of Clinical Neuroscience 1st Faculty of Medicine Charles University and General University Hospital Prague Prague Czechia

Endowment Fund IMPULS Prague Czechia

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