Pharmacokinetics, Safety and Efficacy of Nemolizumab in Participants Aged 6 to 23 Months With Moderate-to Severe Atopic Dermatitis

Inclusion Criteria * Participants greater than and equal to (\>=) 6 to less than (\<) 24 months of age at the screening visit. * Diagnosis of AD according to the American Academy of Dermatology Consensus Criteria at the time of the screening visit. * Eczema Area and Severity Index (EASI) score \>= 16 at both screening and baseline visits. * Investigator's Global Assessment (IGA) score \>= 3 (based on the IGA scale ranging from 0 to 4, in which 3 is moderate and 4 is severe) at both screening and baseline visits. * AD involvement \>= 10 percent (%) of body surface area (BSA) at both screening and baseline visits. (Note: BSA to be derived from the SCORing Atopic Dermatitis (SCORAD).) * Weekly average of worst scratch/itch Numeric Rating Scale (WSI-NRS) score of at least 4 at both screening and baseline visits as assessed by the parent(s)/caregiver: * Screening WSI-NRS score will be determined by a single WSI-NRS assessment (score ranging from 0 to 10) for the 24-hour period immediately preceding the screening visit. * Baseline WSI-NRS score will be determined based on the average of daily WSI-NRS scores (score ranging from 0 to 10) during the 7 days immediately preceding baseline (rounding not permitted). A minimum of 4 daily scores out of the 7 days immediately preceding baseline is required for this calculation. For participants who do not have at least 4 daily scores reported during the 7 days immediately preceding the planned enrollment date, enrollment should be postponed until this requirement is met, without exceeding the maximum screening duration of 4 weeks. * Documented history by a physician and/or investigator of inadequate response to existing topical AD medications or use of systemic therapies for control of the disease (within 6 months before the screening visit). * Participant's parent(s)/caregiver willing and able to comply with all time commitments and procedural requirements of the clinical study protocol. * Participant's parent(s)/caregiver understand and sign a parental Informed Consent Form (ICF) before any study-related procedures are performed. * Participant's parent(s)/caregiver agree to apply a moisturizer daily from the screening visit and liberally as needed throughout the study; if applying a prescribed topical corticosteroid (TCS) prior to study enrollment, agree to continue at the same stable dose from screening and throughout the study as determined appropriate by the investigator. Exclusion Criteria * Body weight \< 5 Kilograms (kg). * Child in Care: a child placed under the control or protection of an agency, organization, institution, or entity by courts or government authorities under applicable law or regulation. * Cutaneous infection within 1 week before the baseline visit or any infection requiring treatment with oral or parenteral antibiotics, antivirals, antiparasitics, or antifungals within 2 weeks before the baseline visit. Participants may be rescreened once the infection has resolved.Note: A participant with mild, localized superficial infection may be included based on investigator discretion. * Having received any of the following treatments within the specified timeframe before the baseline visit: Coal tar products - 2 weeks Topical Janus kinase (JAK) inhibitor - 2 weeks Topical phosphodiesterase-4 (PDE-4) inhibitor - 2 weeks Medium and higher potency TCS - 2 weeks Topical calcineurin inhibitor (TCI) - 2 weeks Topical medications with occlusive dressings (e.g., wet wraps) - 2 weeks Systemic corticosteroids (inhaled and intraocular permitted) - 4 weeks Immunosuppressive or immunomodulatory drugs (e.g., cyclosporine A, oral tacrolimus, cyclophosphamide, azathioprine, methotrexate, mycophenolate mofetil, JAK inhibitors) - 4 weeks or 5 half-lives (whichever is longer) Biologics and biosimilars (e.g., etanercept, adalimumab, infliximab, omalizumab) - 8 weeks or 5 half-lives (whichever is longer) Dupilumab - 10 weeks Live (attenuated) vaccine - 4 weeks Alternative medicine for AD (e.g., traditional Chinese medicine) - 2 weeks For participants with planned live (attenuated) vaccination during the study, consultation with a pediatrician will determine whether vaccination should be postponed until study completion or administered before study start without compromising health. Inactivated vaccines (e.g., diphtheria, tetanus, pertussis (DPT), hepatitis A, hepatitis B, inactivated polio vaccine (IPV), haemophilus influenzae type B, meningococcal, pneumococcal, influenza) are permitted. * Participants who, after a full 16-week treatment course with dupilumab, experienced worsening of AD or failed to achieve minimal improvement (e.g., \<= 10% reduction in EASI or no reduction in IGA). * History of lymphoproliferative disease or malignancy of any organ system before the screening visit. * History of hypersensitivity (including anaphylaxis) to an immunoglobulin product (plasma-derived or recombinant, e.g., monoclonal antibody) or any study treatment excipients. * Known or suspected primary immunodeficiency. * History of, or current confounding skin condition (e.g., Netherton syndrome, psoriasis, contact dermatitis, dermatitis herpetiformis, erythroderma, ichthyosis, scabies) that may interfere with study assessments. * Any medical condition (e.g., serious cardiac, hepatic, pulmonary \[including uncontrolled asthma\], or hematologic disease) or clinically relevant laboratory abnormality during screening that may place the participant at significant risk or interfere with study assessments (e.g., poor venous access or needle phobia). Note: Laboratory tests with abnormal results may be repeated once during screening; the last value will determine eligibility. Uncontrolled asthma defined as one or more of the following: * Asthma exacerbation requiring hospitalization since birth (\<=12 months of age) or within the preceding 12 months (\>12 months of age) * Daytime asthma symptoms \>2 times per week during the preceding 3 months * Nighttime awakenings \>2 times per month during the preceding 3 months * Asthma exacerbation requiring oral corticosteroids or additional Short-Acting * Beta-Agonist (SABA) inhalers \>2 times per week during the preceding 3 months * Planned or expected major surgical procedure during the clinical study. * Planned or anticipated use of prohibited medications during the study. * Currently participating or participated in another clinical study of an investigational drug or device within the past 8 weeks (or 5 half-lives, whichever is longer) before the screening visit, or currently in an exclusion period from a previous clinical study.