Anti-CD20 treatment for multiple sclerosis (MS) blunts the post-COVID-19 humoral immune response, according to new research. The multicentre observational study included 474 patients with MS and related conditions who had a diagnosis of COVID-19. Of the patients not on anti-CD20 treatment, 85% were positive for SARS-CoV-2 antibodies, compared with just 39.5% of patients on anti-CD20 treatment. This statistically significant difference (P < 0.0001) has implications for natural and vaccine-mediated immunity in patients receiving anti-CD20 treatment for MS.