A Study to See How Well Lebrikizumab Works in Adults and Adolescents With Moderate Atopic Dermatitis (Eczema) and High Itch Burden

Inclusion Criteria: * Have chronic atopic dermatitis (AD) (according to American Academy of Dermatology Consensus Criteria) that has been present for ≥1 year before screening visit. * Have 10% to 25% body surface area (BSA) of AD involvement at screening and baseline. * Have pruritus numeric rating scale (NRS) ≥6 at baseline. * Have an Eczema Area and Severity Index (EASI) score ≥16 at screening and baseline. * Have an Investigator's Global Assessment (IGA) score ≥3 (on a scale of 0 to 4) at screening and baseline. * Based on investigator judgement, have a history of inadequate response to treatment with topical medications, or determination that topical treatments are otherwise medically inadvisable. * For participants aged 12 to less than 18, have a body weight ≥40 kilograms (kg) at baseline. Exclusion Criteria: * Have an uncontrolled chronic disease that might require multiple intermittent uses of oral corticosteroids at screening (as defined by the investigator). * Have known liver cirrhosis and/or chronic hepatitis of any etiology. * History of malignancy, including mycosis fungoides or cutaneous T-cell lymphoma, within 5 years before the screening, except completely treated in situ carcinoma of the cervix of completely treated and resolved nonmetastatic squamous or basal cell carcinoma of the skin with no evidence of recurrence in the past 12 weeks. * Are diagnosed with active endoparasitic infections or at high risk of these infections. * Have a known or suspected history of immunosuppression, including history of invasive opportunistic infections despite infection resolution; or unusually frequent, recurrent, or prolonged infections, per the investigator's judgment. * Have presence of skin comorbidities that may interfere with study assessments. * Have a severe concomitant illness(es) that in the investigator's judgment would adversely affect the participant's participation in the study. * Have had any of the following types of infection within 3 months of screening or develop any of these infections during screening: * Serious (requiring hospitalization, and/or intravenous or equivalent oral antibiotic treatment). * Opportunistic - Note: Herpes zoster is considered active and ongoing until all vesicles are dry and crusted over. * Chronic (duration of symptoms, signs, and/or treatment of 6 weeks or longer). * Recurring (including, but not limited to, recurring cellulitis and chronic osteomyelitis). * Have an active or acute infection requiring treatment with systemic antibiotics, antivirals, antiparasitics, antiprotozoals, or antifungals within 2 weeks before the baseline visit or superficial skin infections within 1 week before the baseline visit. * Have had any prior treatment with a biologic therapy for AD. * Have had treatment with any of the following agents within 4 weeks prior to the baseline visit: * systemic immunosuppressive or immunomodulating drugs (for example, systemic corticosteroids, cyclosporine, mycophenolate mofetil, interferon-gamma, * azathioprine, methotrexate, and other immunosuppressants) * small molecules (for example, Janus kinase inhibitors \[topical or systemic\]), or * phototherapy and photochemotherapy for AD. * Treatment with B-cell-depleting biologics, including rituximab, within 6 months prior to baseline.