To find out if eptinezumab is better than placebo (normal saline solution) in lowering the number of days with migraine in young people ages 12 to 17 with chronic migraine.

The study includes a single intravenous (IV) infusion of the study drug and consists of a screening period (4 weeks), a double-blind, placebo-controlled period (12 weeks), and a safety follow-up period (8 weeks). Participants confirmed eligible will be randomized (1:1:1) to receive a single IV infusion of either eptinezumab 300 milligrams (mg) dose (weight adjusted; targeting adult exposure after 300 mg IV), eptinezumab 100 mg (weight adjusted; targeting adult exposure after 100 mg IV), or placebo at randomization visit. The doses will be adjusted for the participant's body weight.

Inclusion Criteria:

• The participant has a diagnosis of migraine (with or without aura) as defined by International Classification of Headache Disorders 3 (ICHD-3) guidelines with history of chronic migraine, of at least 6 months prior to the screening visit.
• During the 28-day screening period, the participant must adequately complete the headache eDiary on at least 23 of the 28 days following the screening visit.
• During the 28-day screening period, the participant must have ≥15 to ≤26 headache days, of which at least 8 are migraine days as documented in the eDiary.

Exclusion Criteria:

• The participant has previously been randomised in this study and exposed to eptinezumab.
• The participant has been exposed to any monoclonal antibody treatment (including exposure in a study) <6 months prior to the screening visit.
• The participant has been exposed to another calcitonin gene-related peptide (CGRP) antibody (including exposure in a study investigating a CGRP antibody) <6 months prior to the screening visit.
• The participant has a history or diagnosis of complicated migraine (ICHD-3 version, 2018), chronic tension-type headache, hypnic headache, cluster headache, hemicrania continua, new daily persistent headache, or unusual migraine subtypes such as hemiplegic migraine (sporadic and familial), migraine with brainstem aura, ophthalmoplegic migraine, or migraine with neurological accompaniments that are not typical of migraine aura (diplopia, altered consciousness, or long duration; for example >60 minutes).