The purpose of this study is to understand the safety and effectiveness of the study drug, Dysport® when compared with placebo in preventing chronic migraine. A migraine is a headache with severe throbbing pain or a pulsating sensation, usually on one side of the head, and is often accompanied by feeling or being sick and a sensitivity to bright lights and sound. Chronic migraine is defined as having at least 15 days of headache a month with at least 8 of those days being migraine headache days. Migraines are caused by a series of events which cause the brain to get stimulated/activated, which results in the release of chemicals that cause pain. Dysport® is a formulation of Botulinum toxin type A (BoNT-A), a medication that stops the release of these chemical messengers. The study will consist of 3 periods: 1. A 'screening period' of 6 to 12 weeks to assess whether the participant can take part to the study and requires 1 visit. 2. A first Treatment Phase of 24 weeks. On Day 1 and at Week 12 of the first Treatment Phase, participants will receive injections into various muscles across the head, neck, face and shoulders. The injections will contain either a dose "A" or dose "B" of Dysport® or a placebo (an inactive substance or treatment that looks the same as, and is given in the same way as, an active drug or intervention/treatment being studied). Participants will make 4 visits to the clinic in person and have 4 remote (online) visits. 3. A second Treatment Phase of 24 weeks (extension phase). At Week 24 and at Week 36, all participants will get Dysport® (dose "A" or dose "B"). There will be 3 in person visits and 4 remote visits. Participants will need to complete an e-diary and questionnaires throughout the study. Participants will undergo blood samplings, urine collections, physical examinations, and clinical evaluations. They may continue some other medications, but the details need to be recorded. The total study duration for a participant will be up to 60 weeks (approx. 14 months).
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Change from baseline at week 24 in monthly migraine days (MMD)
Timeframe: Every 4 weeks from Week 4 (Weeks 1-4) to Week 24 (Weeks 21-24)