A migraine day was any calendar day in which the participant experienced a qualified migraine headache (onset, continuation, or recurrence of the migraine headache). A qualified migraine headache was defined either as a migraine with or without aura.

The change from baseline in monthly migraine days was calculated as the number of migraine days during the last 4 weeks of the 12-week double-blind treatment phase - the number of migraine days during the 4-week baseline phase.

The analysis was conducted in the efficacy analysis set which includes participants who received at least 1 dose of study drug and had at least 1 change from baseline measurement in monthly migraine days in the double-blind treatment phase.

A migraine day was any calendar day in which the participant experienced a qualified migraine headache (onset, continuation, or recurrence of the migraine headache). A qualified migraine headache was defined either as a migraine without aura or a migraine with aura. Monthly migraine days were calculated as the number of migraine days in the 4-week baseline phase and during the last 4 weeks of double-blind treatment.

At least a 50% reduction from baseline in monthly migraine days was determined if the change in monthly migraine days from the 4-week baseline phase to the last 4 weeks of the 12-week double-blind treatment phase * 100 / baseline monthly migraine days was less than or equal to -50%.

The analysis was conducted in the efficacy analysis set which includes participants who received at least 1 dose of study drug and had at least 1 change from baseline measurement in monthly migraine days in the double-blind treatment phase. Participants with missing data at week 12 were counted as non-responders.

Monthly acute migraine-specific medication treatment days is the number of days on which migraine specific medications were used between monthly doses of study drug. Migraine-specific medications includes two categories of medications: triptan-based migraine medications and ergotamine-based migraine medications.

The change from baseline in monthly acute migraine-specific treatment days was calculated as the number of migraine-specific treatment days during the last 4 weeks of the 12-week double-blind treatment phase - the number of migraine-specific treatment days during the 4-week baseline phase.

The analysis was conducted in the efficacy analysis set which includes participants who received at least 1 dose of study drug and had at least 1 change from baseline measurement in monthly acute migraine-specific treatment days in the double-blind treatment phase.

The Migraine Physical Function Impact Diary (MPFID) is a self-administered 13-item instrument measuring physical functioning. It has two domains, Impact on Everyday Activities (7 items) and Physical Impairment (5 items), and one stand-alone global question. Participants completed the MPFID daily in an electronic diary based on the past 24 hours. Participants responded to each item on a 5-point scale, with difficulty items ranging from "Without any difficulty" (1) to "Unable to do" (5) and frequency items ranging from "None of the time" (1) to "All of the time" (5). For each domain, the scores were calculated as the sum of the responses and rescaled to 0 - 100, with higher scores representing greater impact of migraine.

Achievement of at least a 5 point reduction from baseline in the monthly average domain score was calculated as (monthly average domain score during the last 4 weeks of the 12-week double-blind treatment phase - baseline monthly average domain score) was ≤ -5.

The analysis was conducted in the efficacy analysis set including participants who received at least 1 dose of study drug and had at least 1 change from baseline measurement in MPFID average impact on everyday activities domain score in the double-blind treatment phase. Participants with missing post-baseline data were counted as non-responders.

The Migraine Physical Function Impact Diary (MPFID) is a self-administered 13-item instrument measuring physical functioning. It has two domains, Impact on Everyday Activities (7 items) and Physical Impairment (5 items), and one stand-alone global question. Participants completed the MPFID daily in an electronic diary based on the past 24 hours. Participants responded to each item on a 5-point scale, with difficulty items ranging from "Without any difficulty" (1) to "Unable to do" (5) and frequency items ranging from "None of the time" (1) to "All of the time" (5). For each domain, the scores were calculated as the sum of the responses and rescaled to 0 - 100, with higher scores representing greater impact of migraine.

Achievement of at least a 5 point reduction from baseline in the monthly average domain score was calculated as (monthly average domain score during the last 4 weeks of the 12-week double-blind treatment phase - baseline monthly average domain score) was ≤ -5.

The analysis was conducted in the efficacy analysis set including participants who received at least 1 dose of study drug and had at least 1 change from baseline measurement in MPFID average impact on physical impairment domain score in the double-blind treatment phase. Participants with missing post-baseline data were counted as non-responders.

Adverse events (AEs) were graded according to the Common Terminology Criteria for Adverse Events (CTCAE), version 4, where:

Grade 1 = Mild; asymptomatic or mild symptoms; clinical or diagnostic observations only; intervention not indicated; Grade 2 = Moderate; minimal, local or noninvasive intervention indicated; limiting age-appropriate instrumental activities of daily living (ADL); Grade 3 = Severe or medically significant but not immediately life-threatening; hospitalization or prolongation of hospitalization indicated; disabling; limiting self care ADL; Grade 4 = Life-threatening consequences; urgent intervention indicated Grade 5 = Death related to AE.

For the double-blind treatment phase adverse events were analyzed for all randomized participants who received at least one dose of study drug. For the open-label treatment phase adverse events were analyzed for all participants who received at least one dose of study drug in the open-label treatment phase.

Blood samples were first tested in an electrochemiluminescence (ECL)-based bridging immunoassay to detect anti-drug antibodies (ADA) against erenumab. Samples confirmed to be positive for binding antibodies were subsequently tested in a cell-based bioassay to determine neutralizing activity against erenumab (Neutralizing Antibody Assay).

Developing antibody incidence indicates participants with a negative or no result at baseline and a positive result at any time post-baseline.

If a sample was positive for binding antibodies and demonstrated neutralizing activity at the same time point, the sample was defined as positive for neutralizing antibodies.

Transient indicates a negative result at the participant's last time point tested, for those participants with a positive binding/neutralizing result post-baseline.

Participants who received at least one dose of erenumab and with post-baseline data are included in the analysis.