COVID-19 and the Risk of Relapse in Multiple Sclerosis Patients: A Fight with No Bystander Effect?

BACKGROUND: COVID-19 is speculated to increase the likelihood of relapsing-remitting multiple sclerosis (RRMS) exacerbation. OBJECTIVE: To investigate the association between contraction of COVID-19 and incidence of acute MS attacks in RRMS patients six months post-infection. METHODS: This retrospective cohort study compares the risk of relapse in RRMS patients with (n=56) and without COVID-19 (n=69). Incidence of relapse was recorded for six-month following contraction of COVID-19. Incidence of RRMS exacerbation in patients with COVID-19 was compared to patients without COVID-19 (the independent control group) and the same patients six months prior to the COVID-19 pandemic. RESULTS: A lower incidence rate of RRMS exacerbation was observed in patients that contracted COVID-19 than in patients who did not contract COVID-19 (incidence rate ratio: 0.275; p=0.026). Self-controlled analysis showed no significant difference in relapse rates before the COVID-19 pandemic and after contracting COVID-19 (p=0.222). The relapse risk was not different between patients who had been hospitalized due to COVID-19 severity and those who had not (p=0.710). CONCLUSION: COVID-19 contraction may not increase the risk of acute MS attacks shortly following contraction. We hypothesize that COVID-19-associated lymphopenia may partly preclude the autoreactive memory cells from expansion and initiating relapses through a so-called bystander effect of COVID-19 infection.