A Study to Evaluate the Pharmacokinetics, Safety, and Effectiveness of Certolizumab Pegol in Children With Moderate to Severe Chronic Plaque Psoriasis

Inclusion Criteria: * Study participant must have a diagnosis of moderate to severe plaque psoriasis (PSO) for ≥3 months and: 1. Body Surface Area (BSA) affected by psoriasis ≥10 % 2. Physician's Global Assessment (PGA) score ≥3 (on a scale from 0 to 4) 3. Psoriasis Area and Severity Index (PASI) score is ≥12 or 4. PASI score is ≥10 and \<12 with at least one of the following: * \>Clinically relevant facial or scalp involvement * \>Clinically relevant genital involvement * \>Clinically relevant palm and sole involvement * \>Clinically relevant axillary involvement Study participants aged ≥12 years may alternatively have a diagnosis of moderate to severe mixed guttate/plaque PSO with \>50 % to \<80 % guttate lesions for ≥3 months, and must meet the same criteria listed above * Study participant must be a candidate for systemic psoriasis therapy and/or phototherapy and/or photochemotherapy Exclusion Criteria: * Study participant previously participated in this study or has previously been treated with certolizumab pegol (CZP) * Study participant has generalized pustular or erythrodermic psoriasis (PSO) * Study participant has guttate PSO without plaque PSO * Study participant has had a primary failure to an anti-tumor necrosis factor agent * Study participant has had prior exposure to \>2 biologic therapies * Study participant has a history of severe major depression or suicide attempt (including an actual attempt, interrupted attempt, or aborted attempt), or has had suicidal ideation in the past 6 months as indicated by a positive response ("Yes") to either Question 4 or Question 5 of the "Screening/Baseline" version of the Columbia Suicide Severity Rating Scale (CSSRS) at Screening