Background Natalizumab (NTZ) and ocrelizumab (OCR) can be used for the treatment of relapsing–remitting multiple sclerosis (RRMS). In patients treated with NTZ, screening for JC virus (JCV) is mandatory, and a positive serology usually requires a change in treatment after 2 years. In this study, JCV serology was used as a natural experiment to pseudo-randomize patients into NTZ continuation or OCR. Methods An observational analysis of patients who had received NTZ for at least 2 years and were either changed to OCR or maintained on NTZ, depending on JCV serology status, was performed. A stratification moment (STRm) was established when patients were pseudo-randomized to either arm (NTZ continuation if JCV negativity, or change to OCR if JCV positivity). Primary endpoints include time to first relapse and presence of relapses after STRm and OCR initiation. Secondary endpoints include clinical and radiological outcomes after 1 year. Results Of the 67 patients included, 40 continued on NTZ (60%) and 27 were changed to OCR (40%). Baseline characteristics were similar. Time to first relapse was not significantly different. Ten patients in the JCV + OCR arm presented a relapse after STRm (37%), four during the washout period, and 13 patients in the JCV-NTZ arm (32.5%, p = 0.701). No differences in secondary endpoints were detected in the first year after STRm. Conclusions The JCV status can be used as a natural experiment to compare treatment arms with a low selection bias. In our study, switching to OCR versus NTZ continuation led to similar disease activity outcomes.

<b><i>Introduction:</i></b> Prior studies have suggested that cardiovascular risk factors (CVRFs) can affect the prognosis of multiple sclerosis (MS). The aim of this study was to assess if CVRFs affect the early course of MS. <b><i>Methods:</i></b> A retrospective observational study was performed, including patients diagnosed with relapsing-remitting MS (RRMS) from 2010 to 2020, with at least 2 years of disease and 6 months follow-up. Age at onset, disease duration, number of relapses, time to confirmed Expanded Disability Status Scale (EDSS) 3.0 and 6.0, and time to secondary progressive MS (SPMS) were collected. Presence and date at onset of hypertension (HT), diabetes mellitus (DM), high low-density lipoprotein cholesterol (LDLc), and smoking during the study period were collected. The primary objective was to assess if CVRFs at the onset of MS are associated with lower time to EDSS 3.0, time to EDSS 6.0, and time to SPMS, using bivariate and multivariate analysis. <b><i>Results:</i></b> 281 RRMS patients were included; median age at onset was 33 (IQR 26–39); 69.4% were female. Median EDSS at onset was 1.5 (IQR 1–2.5). Nine patients reached SPMS; 24 patients were diagnosed with HT, 9 with DM, 109 with high LDLc, and 123 were smokers during follow-up. No statistically significant association was found between the presence of CVRF at MS onset and the mentioned clinical outcomes during the MS course. <b><i>Conclusion:</i></b> No association was found between CVRFs and the early course of MS in our cohort.