The purpose of this study is to evaluate how well LY4005130 works in participants with severe alopecia areata (hair loss) when compared with placebo, and how well it's tolerated and what side effects may occur. Blood tests will be performed to investigate how the body processes the study drug and how the study drug affects the body.

The study drug will be administered intravenously (IV) (into a vein in the arm).

The study will last approximately 48 weeks, including screening.

Inclusion Criteria:

• Have severe Alopecia Areata (AA) that meets all of the following criteria:

  • Hair loss encompassing ≥50% and ≤90% of the scalp, as measured by Severity of Alopecia Tool (SALT) score
  • The duration of the current episode of severe AA is at least 6 months and does not exceed 4 years
  • No significant spontaneous hair regrowth in the investigator's opinion for at least 6 months
  • Agree not to use any AA treatments during the study

Exclusion Criteria:

• Primarily "diffuse" type of AA (characterized by diffuse hair shedding)
• Are currently experiencing other forms of alopecia
• Participants who, in the opinion of the investigator, are currently experiencing or have a history of unstable concomitant disease that requires frequent hospitalizations, and/or frequent use of systemic immunosuppressants that may interfere with participation in the study
• Have received oral JAK Inhibitors in the past
• Have had any major surgery within 8 weeks prior to screening or will require major surgery during the study
• Have a history or presence of cardiovascular, respiratory, hepatic, gastrointestinal, endocrine, hematological, neurological, or neuropsychiatric disorders or any other serious and/or unstable illness

Anonymized individual patient level data will be provided in a secure access environment upon approval of a research proposal and a signed data sharing agreement.

Data are available 6 months after the primary publication or approval of the indication studied in the US and European Union (EU), whichever is later. Data will be indefinitely available for requesting.

A research proposal must be approved by an independent review panel and researchers must sign a data sharing agreement.