Short report: Real-life analysis of the occurrence of persistent, transient, and fluctuating positive titres of neutralizing anti-drug antibodies in multiple sclerosis patients treated with interferon beta

•Transient and fluctuating patterns of NAbs against interferon-β in MS are rare.•Patients with transient patterns of NAbs generally present with low titres.•In contrast, patients with fluctuating NAbs commonly have medium or high titres.•The cause of these patterns is not elicudated, but require further investigation.•NAbs can still be key in patients with disease flares despite long treatment times. Interferon beta (IFNβ) is a first line therapy for treatment of multiple sclerosis (MS). However, up to 47% of treated patients will develop neutralizing anti-drug antibodies (NAbs) against IFNβ, which at high titres can inhibit the therapeutic effect of the drug. This study aimed to determine the frequency of transient and fluctuating NAb positivity in a real-world clinical routine setting using a large retrospective international cohort of IFNβ-treated MS patients collected as a part of the ABIRISK consortium (n = 9657). Transient and fluctuating NAbs were rare (2.6% and 0.9%, respectively), but bring noteworthy considerations about clinical decisions in context of NAbs.