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 h…