A Study to Evaluate the Efficacy and Safety of Bimekizumab Compared to Ustekinumab in Children and Adolescents From 6 Years to Less Than 18 Years of Age With Moderate to Severe Plaque Psoriasis

Inclusion Criteria: * Study participant must be 6 to \<18 years of age, inclusive, at the time of signing the informed consent/assent according to local regulation * Study participant has had a diagnosis of moderate to severe plaque psoriasis (PSO) for at least 3 months prior to the Screening Visit * Study participant meets the following at both the Screening and Baseline Visits: 1. Body surface area (BSA) affected by PSO ≥10% 2. . Investigator's Global Assessment (IGA) score ≥3 (on a scale from 0 to 4) 3. . Psoriasis Area and Severity Index (PASI) score ≥12 OR PASI score ≥10 plus at least 1 of the following: i) Clinically relevant facial involvement ii) Clinically relevant genital involvement iii) Clinically relevant hand and foot involvement * Study participant is a candidate for systemic PSO therapy and/or photo/chemotherapy and for treatment with ustekinumab per labeling * Study participant has body weight ≥15 kg and body mass index for age percentile of ≥5 at Screening Exclusion Criteria: * Primary failure (no response within 12 weeks) to 1 or more interleukin-17 (IL-17) biologic response modifiers (eg, brodalumab, ixekizumab, secukinumab) OR more than 1 biologic response modifier other than an IL-17 * Study participant has a presence of guttate, inverse, pustular, or erythrodermic PSO or other dermatological condition that may impact the clinical assessment of PSO * Study participant has a history of inflammatory bowel disease (IBD) or symptoms suggestive of IBD * History of active tuberculosis unless successfully treated, latent TB unless prophylactically treated * Study participant has an active infection or history of infections (such as serious infection, chronic infections, opportunistic infections, unusually severe infections) * Study participant has previously received bimekizumab * Study participant has previously received ustekinumab * Study participant has received drugs outside the specified timeframes relative to the Baseline Visit or receives prohibited concomitant treatments * Study participant has the presence of active suicidal ideation, or positive suicide behavior * Study participant diagnosed with severe depression in the past 6 months (prior to Screening) should be excluded * Study participant has a history of psychiatric inpatient hospitalization within the past year before enrolling into the study