Interplay between age and disease-modifying treatments in influencing infection risk in multiple sclerosis.

Background: Disease-modifying treatments (DMTs) can increase the risk of infections in multiple sclerosis (MS). Aged individuals are usually excluded from clinical trials, and there is uncertainty regarding safety of immunosuppressive DMTs in these patients.

Objective: To investigate the association of DMTs, ageing and other clinical variables with risk of infections in MS patients.

Methods: Prospective single-centre observational study collecting information on occurrence, type and grade of infections in patients followed at the MS centre, Lugano (Switzerland). Associations with infection risk were tested using multivariable Poisson and Cox regressions.

Results: A total of 503 patients were included (injectables/untreated, = 127; orals, = 139; monoclonal antibodies (MAB), = 237) and 326 infections recorded over 12.6 (11.6-14.0) months. As compared to injectable DMTs/no treatment, MAB and oral DMTs were positively associated with infection incidence (IRR = 2.32, 95% confidence interval (CI) = 1.39-3.89, = 0.001; IRR = 2.04, 95% CI = 1.19-3.49, = 0.009, respectively). After excluding COVID-19, the effect of MAB was stronger among patients <50 years (IRR = 5.90, 95% CI = 2.80-12.45, < 0.001) than >50 years (IRR = 1.95, 95% CI = 0.91-4.15, = 0.084). Higher disability and male sex were the only variables associated with severe infections.

Conclusion: Treatment with MAB and oral DMTs is associated with higher incidence of infections, with a stronger effect in young MS patients. Disability appears the main predictor of severe infections regardless of treatment.