To evaluate the effect of erenumab compared with placebo on the change in MMDs from baseline to week 9 through week 12 (month 3) of DBTP.

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, lasting for ≥ 4 hours, and meeting at least 1 of the following criteria: 1. ≥ 2 of the following pain features: * unilateral * throbbing * moderate to severe * exacerbated with exercise/physical activity 2. ≥ 1 of the following associated symptoms: * nausea * vomiting * photophobia and phonophobia The change from baseline in monthly migraine days was calculated as the average number of migraine days per month during the last 3 months (months 4, 5, and 6) of the 24-week double-blind treatment period minus the number of migraine days during the 4-week baseline period.

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.

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 average number of migraine days per month during the last 3 months (months 4, 5, and 6) of the 24-week double-blind treatment phase - the number of migraine days during the 4-week baseline phase.

A migraine day was defined as 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 a migraine with or without aura, lasting for ≥ 30 minutes, and meeting at least one of the following criteria: a) ≥ 2 of the following pain features: unilateral, throbbing, moderate to severe, exacerbated with exercise/physical activity; b) ≥ 1 of the following associated symptoms: nausea and/or vomiting, photophobia and phonophobia. Change from baseline was calculated using the mean monthly migraine days from months 4, 5 and 6 of the DBTP minus the number of migraine days during the 4-week baseline phase. Least squares (LS) mean was estimated using a generalized linear mixed model which included treatment, visit, treatment by visit interaction, stratification factor (current, prior only, or no prior/current treatment), and baseline value as covariates.

On CHU day 29 and day 57, site staff interviewed sub-study participants and asked if they administered a full, partial, or no dose of erenumab (after explaining that a full dose means that the entire volume of the AI/pen was injected) and documented the participant's response in the electronic case report form. Data presented are the percentage of participants who reported "full administration," "not full administration," or "discontinued investigational product (ie, no dose)." (Day 1 of the CHU substudy occurred at any OLTP study visit \[up to week 88\] as long as the participant had previously received at least 1 OL dose of erenumab 140 mg.)

To assess participants ability to administer a full dose of erenumab in home-use at day 29 (week 4) and day 57 (week 8), site staff called participants and asked whether the participant administered a full, partial, or no dose of erenumab. A full dose means that the entire volume of both prefilled syringes or autoinjector/pens were injected. Discontinued prior to dosing day indicates participants who had discontinued the investigational product and did not attempt to self-administer on day 29 or 57.

On day 1 of the double-blind randomization phase participants received 140 mg intravenous erenumab over 30 minutes or matching placebo. On day 8, participants received 10 mol/kg/minute PACAP-38 over 10 minutes and were observed for 24 hours after PACAP-38 infusion. A MLA was defined as fulfilling 1 of the 2 criteria: 1. Headache with at least 2 of the following characteristics: unilateral location, pulsating quality, moderate or severe pain intensity, aggravated by/causing avoidance of routine physical activity. Additionally, during the headache at least 1 of the following: nausea and/or vomiting, photophobia or phonophobia. 2. Headache described as mimicking usual migraine attack treated with triptan.

An adverse event (AE) is defined as any untoward medical occurrence in a clinical trial participant. The event does not necessarily have a causal relationship with study treatment. The definition of adverse events includes worsening of a pre-existing medical condition. Laboratory value changes that require treatment or adjustment in current therapy are considered adverse events. Teatment-related adverse events (TRAEs) are those assessed by the investigator as being possibly related to study drug. A serious adverse event is defined as an adverse event that meets at least 1 of the following serious criteria: * fatal * life-threatening (places the subject at immediate risk of death) * requires in-patient hospitalization or prolongation of existing hospitalization * results in persistent or significant disability/incapacity * congenital anomaly/birth defect * other medically important serious event.

The C-SSRS is a measure of suicidal ideation and behavior. Ideation includes a wish to be dead or nonspecific thoughts about wanting to end life. Suicidal behavior includes actual attempts, interrupted or aborted attempts, and any preparatory acts.

Participants who had a negative or no result at baseline and were antibody positive postbaseline. Blood samples were first tested for anti-erenumab binding antibodies, samples testing positive for binding antibodies were also tested for neutralizing antibodies.