Long-term Safety and Efficacy Study of Lebrikizumab (LY3650150) in Participants With Moderate-to-Severe Atopic Dermatitis (ADjoin)

Inclusion Criteria for participants coming from a parent study: Participants must meet all the following criteria to be eligible for this study: * Received treatment in a lebrikizumab study, NCT04146363, NCT04178967, NCT04250337, NCT04250350 and have adequately completed the study treatments and last patient visit of the parent trial. * For women of childbearing potential: agree to remain abstinent (refrain from heterosexual intercourse) or use a highly effective contraceptive method during the treatment period and for at least 18 weeks after the last dose of lebrikizumab or placebo. Exclusion Criteria for participants coming from a parent study: Participants meeting any of the criteria below will not be included in this study: * Participants who, during their participation in the parent trial, developed a serious adverse event (SAE) deemed related to lebrikizumab, which in the opinion of the investigator or of the medical monitor could indicate that continued treatment with lebrikizumab may present an unreasonable risk for the participant. \* * Participants who, during their participation in the parent trial, developed an AE that was deemed related to lebrikizumab and led to study treatment discontinuation, which in the opinion of the investigator or of the medical monitor could indicate that continued treatment with lebrikizumab may present an unreasonable risk for the participant. \* * Conditions in the previous parent study consistent with protocol-defined criteria for permanent study drug discontinuation, if deemed related to lebrikizumab or led to investigator - or sponsor-initiated withdrawal of participant from the study (e.g., non-compliance, inability to complete study assessments, etc.). \* * Pregnant or breastfeeding women, or women planning to become pregnant or breastfeed during the study. Open-Label Addendum Inclusion Criteria: Participants must meet all the following criteria to be eligible for this study addendum: * Male or female adults and adolescents (≥12 to \<18 years of age and weighing ≥40 kilogram (kg). * Chronic AD (according to American Academy of Dermatology Consensus Criteria) that has been present for ≥1 year before screening. * Eczema Area and Severity Index (EASI) score ≥16 at baseline. * Investigator Global Assessment (IGA) score ≥3 (scale of 0 to 4) at baseline. * ≥10% body surface area (BSA) of AD involvement at baseline. * History of inadequate response to treatment with topical medications; or determination that topical treatments are otherwise medically inadvisable. Open-Label Addendum Exclusion Criteria: Participants meeting any of the criteria below will be excluded from this study addendum: * Have received a dose of lebrikizumab in any prior lebrikizumab clinical study. * History of anaphylaxis * Treatment with topical prescription moisturizers, corticosteroids, calcineurin inhibitors, or phosphodiesterase-4 inhibitors such as crisaborole within 1 week prior to baseline. * Treatment with any of the following agents within 4 weeks prior to the baseline. * Immunosuppressive/immunomodulating drugs (e.g., systemic corticosteroids, cyclosporine, mycophenolate-mofetil, Interferon gamma (IFN-γ), Janus kinase inhibitors, azathioprine, methotrexate, etc.) * Phototherapy and photochemotherapy (PUVA) for AD. * Treatment with the following prior to baseline: * Immunosuppressive/immunomodulating drugs (e.g., systemic corticosteroids, cyclosporine, mycophenolate-mofetil, IFN-γ, Janus kinase inhibitors, azathioprine, methotrexate, etc.) * Phototherapy and photochemotherapy (PUVA) for AD. * Treatment with the following prior to baseline: * An investigational drug within 8 weeks or within 5 half-lives (if known), whichever is longer. * B Cell-depleting biologics, including rituximab, within 6 months. * Other Biologics within 5 half-lives (if known) or 8 weeks, whichever is longer. * Regular use (more than 2 uses per week) of a tanning booth/parlor within 4 weeks of baseline. * Treatment with a live (attenuated) vaccine within 12 weeks of the baseline or planned during the study. * Uncontrolled chronic disease that might require multiple intermittent uses of oral corticosteroids, e.g., co-morbid severe uncontrolled asthma, (as defined by the investigator). * Pregnant or breastfeeding women, or women planning to become pregnant or breastfeed during the study. * Evidence of active acute or chronic hepatitis * History of human immunodeficiency virus (HIV) infection or positive HIV serology at screening. * History of malignancy, including mycosis fungoides, within 5 years before screening, except completely treated in situ carcinoma of the cervix, completely treated and resolved non-metastatic squamous or basal cell carcinoma of the skin with no evidence of recurrence in the past 12 weeks. Open-Label Extension Addendum Inclusion Criteria: Have completed Week 100 of Study KGAA and have not yet completed the safety follow-up visit for the main study. Open-Label Extension Addendum Exclusion Criteria: Have initiated treatment with a medication prohibited by Study KGAA before addendum baseline. This includes use of biologics for AD (for example, dupilumab and tralokinumab) during the safety follow-up period of Study KGAA.