BACKGROUND: When the novel coronavirus disease 2019 (COVID-19) appeared, concerns about its course in patients with multiple sclerosis (MS) and neuromyelitis optica spectrum disorder (NMOSD) arose. This study aimed to evaluate the incidence, severity and risk factors of the more severe COVID-19 course among MS and NMOSD patients. METHODS: From March 1, 2020, to February 28, 2021, 12 MS centres, representing 70% of the Czech MS and NMOSD population, reported laboratory-confirmed COVID-19 cases via the Czech nationwide register of MS and NMOSD patients (ReMuS). The main outcome was COVID-19 severity assessed on an 8-point scale with a cut-off at 4 (radiologically confirmed pneumonia) according to the World Health Organisation s (WHO) COVID-19 severity assessment. RESULTS: We identified 958 MS and 13 NMOSD patients, 50 MS and 4 NMOSD patients had pneumonia, 3 MS and 2 NMOSD patients died. The incidence of COVID-19 among patients with MS seems to be similar to the general Czech population. A multivariate logistic regression determined that higher body mass index (BMI [OR 1.07, 95% CI, 1.00-1.14]), older age (OR per 10 years 2.01, 95% CI, 1.41-2.91), high-dose glucocorticoid treatment during the 2 months before COVID-19 onset (OR 2.83, 95% CI, 0.10-7.48) and anti-CD20 therapy (OR 7.04, 95% CI, 3.10-15.87) were independent variables associated with pneumonia in MS patients. Increase odds of pneumonia in anti-CD20 treated MS patients compared to patients with other disease-modifying therapy (same age, sex, BMI, high-dose glucocorticoid treatment during the 2 months before COVID-19 onset, presence of pulmonary comorbidity) were confirmed by propensity score matching (OR 8.90, 95% CI, 3.04-33.24). Reports on COVID-19 infection in patients with NMOSD are scarce, however, data available up to now suggest a high risk of a more severe COVID-19 course as well as a higher mortality rate among NMOSD patients. In our cohort, 4 NMOSD patients (30.77%) had the more severe COVID-19 course and 2 patients (15.39%) died. CONCLUSION: The majority of MS patients had a mild COVID-19 course contrary to NMOSD patients, however, higher BMI and age, anti-CD20 therapy and high-dose glucocorticoid treatment during the 2 months before COVID-19 onset were associated with pneumonia. Based on this study, we have already started an early administration of anti-SARS-CoV-2 monoclonal antibodies and preferential vaccination in the risk group of patients.