Safety, Pharmacokinetics and Efficacy of Dupilumab in Patients ≥6 Months to <6 Years With Moderate-to-Severe Atopic Dermatitis (Liberty AD PRESCHOOL)

1. Part A (open-label, single-ascending-dose, sequential cohort phase 2 study): * Primary objective is to characterize the safety and PK of dupilumab administered as a single dose in pediatric participants, 6 months to less than 6 years of age, with severe AD. * Secondary objective is to evaluate the efficacy and immunogenicity of a single dose of dupilumab in participants 6 months to less than 6 years of age with severe AD. 2. Part B (randomized, double-blind, parallel-group, placebo-controlled phase 3 study): * Primary objective is to demonstrate the efficacy of multiple doses of dupilumab over 16 weeks of treatment when administered concomitantly with topical corticosteroids (TCS) in pediatric participants, 6 months to less than 6 years of age, with moderate-to-severe AD. * Secondary objective is to assess the safety and immunogenicity of multiple doses of dupilumab over 16 weeks of treatment when administered concomitantly with TCS in participants 6 months to less than 6 years of age with moderate-to-severe AD.