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 treatment phase - the number of migraine days during the 4-week baseline phase.

The efficacy analysis set included participants who received at least 1 dose of study drug and completed at least 1 post-baseline monthly eDiary measurement. The number of participants analyzed includes those with observed data at week 12.

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 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 treatment phase * 100 / baseline monthly migraine days was less than or equal to -50%.

The efficacy analysis set included participants who received at least 1 dose of study drug and completed at least 1 post-baseline monthly eDiary measurement.

Participants with missing post-baseline data 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 efficacy analysis set included participants who received at least 1 dose of study drug and completed at least 1 post-baseline monthly eDiary measurement.

The number of participants analyzed includes those with observed data at week 12.

The cumulative duration of any qualified headache between monthly doses of study drug regardless of acute treatment use.

A qualified headache was defined as follows:

• a qualified migraine headache (including an aura-only event that is treated with acute migraine-specific medication), or
• a qualified non-migraine headache, which is a headache that lasted continuously for ≥ 4 hours and was not a qualified migraine headache, or
• a headache of any duration for which acute headache treatment was administered.

The efficacy analysis set included participants who received at least 1 dose of study drug and completed at least 1 post-baseline monthly eDiary measurement.

The number of participants analyzed includes those with observed data at week 12.

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.

All randomized participants who received at least one dose of study drug.

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.

Randomized participants who received at least one dose of study drug and with available post-baseline antibody data. This endpoint was analyzed in the erenumab treatment groups only.