A Study to Evaluate the Efficacy, Safety, and Tolerability of Brivaracetam as Monotherapy in Patients 2 to 25 Years of Age With Childhood Absence Epilepsy or Juvenile Absence Epilepsy

Inclusion Criteria: * Study participant is 2 to 25 years of age inclusive, at the time of signing the informed consent. No study participants from 2 to \<4 years of age will be included in Stage 1 * Study participant is diagnosed with either childhood absence epilepsy (CAE) or juvenile absence epilepsy (JAE) as defined by the International League Against Epilepsy (ILAE) criteria * Study participants 2 to \<4 years of age and participants who had onset of absence seizures at an age younger than 4 years must have a negative glucose transporter type 1 deficiency syndrome (GLUT1DS) genetic test * Study participant is untreated with antiepileptic drugs (AEDs) or pretreated for absence seizures with a maximum of 2 historical AEDs, but without AED treatment for a period of at least 5 half-lives of the AED before randomization into this study. The UCB study physician should be consulted if in doubt * Study participant has electroencephalogram (EEG) evidence of bilateral synchronous, symmetric generalized paroxysmal spike waves (2.5-6 hertz) with normal background activity and with at least 1 electrographically recorded seizure lasting 3 seconds or more on a 1-hour EEG with hyperventilation (HV) while awake at Visit 1 (V1), or on a historical EEG up to 12 weeks before enrollment * Study participant has a history of clinically evident absence seizures occurring on at least 3 days per week in the 2 weeks prior to enrollment * Study participant is without treatment with psychoactive drugs or on a stable dose for at least 2 weeks prior to randomization * Study participant has normal neurological examination, head size, development and cognition * Body weight is ≥9 kg * Male and female a) A sexually active male study participant must agree to use contraception during the treatment period and for at least 2 days, corresponding to the time needed to eliminate study treatment, after the last dose of study treatment and refrain from donating sperm during this period b) A female study participant is eligible to participate if she is not pregnant, not breastfeeding, and at least 1 of the following conditions applies: The study participant is premenarchial OR A woman of childbearing potential (WOCBP) who agrees to follow the contraceptive guidance during the treatment period and for at least 2 days after the last dose of study treatment, corresponding to the time needed to eliminate study treatment * Study participant provides consent/assent, and the study participant's parent/legal representative/caregiver provides signed informed consent for minor study participants, which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol Exclusion Criteria: * Study participant has a history of nonfebrile seizures other than absence seizures (eg, generalized tonic-clonic seizures or myoclonic seizures) * Study participant has a history of absence status epilepticus * Study participant has a history or presence of paroxysmal nonepileptic seizures * Study participant has a clinically relevant electrocardiogram (ECG) abnormality in the opinion of the Principal Investigator * Study participant has hepatic impairment (Child Pugh Score A, B, or C) based on the Investigator's assessment * Study participant has a history of major psychiatric disease or any clinically significant medical condition that would preclude appropriate study participation * Study participant has active suicidal ideation prior to study entry as indicated by a positive response ("Yes") to either Question 4 or Question 5 of the Columbia-Suicide Severity Rating Scale (C-SSRS; for study participants 6 years or older) or clinical judgement (for study participants younger than 6 years). The study participant should be referred immediately to a Mental Healthcare Professional * Study participant has a lifetime history of suicide attempt (including an active attempt, interrupted attempt, or aborted attempt). The study participant should be immediately evaluated by a Mental Healthcare Professional to address safety concerns * Study participant with known fructose intolerance or hypersensitivity of any of the ingredients in brivaracetam oral solution * Study participant has end-stage kidney disease requiring dialysis * Concomitant use of rifampicin/rifampin; prior use must have been stopped at least 2 months before randomization * Concomitant use of strong CYP2C19 inhibitors like fluconazole, fluoxetine and fluvoxamine, prior use must have been stopped at least 1 week before randomization * Study participant has participated in another study of an investigational medicinal product (IMP; and/or an investigational device) within the previous 30 days prior to informed consent * Study participant has clinical or EEG findings not consistent with a diagnosis of childhood absence epilepsy (CAE) or juvenile absence epilepsy (JAE)