Inclusion Criteria: * Participant must be aged between ≥6 months and \<36 months in Part 1A, ≥48 months and \<18 years in Part 1B, ≥6 months and \<48 months in Part 2. * Participant must have a predicted loss of function pathogenic or likely pathogenic SCN1A variant. * Participant must have experienced their first seizure between the ages of 3 and 15 months. * Participant must have a clinical diagnosis of Dravet syndrome or the treating clinician must have a high clinical suspicion of a diagnosis of Dravet syndrome. * Participant is receiving at least one prophylactic antiseizure medication. Exclusion Criteria: * Participant has another genetic mutation or clinical comorbidity which could potentially confound the typical Dravet phenotype. * Participant has a known central nervous system structural and/or vascular abnormality (indicated by an MRI or CT scan of the brain). * Participant has an abnormality that may interfere with CSF distribution and/or has an existing ventriculoperitoneal shunt. * Participant has received sodium channel blockers during the Pre-Dosing Seizure Period. * Participant has experienced seizure freedom for a period of 4 consecutive weeks within the 90-day period prior to informed consent. * Participant has previously received gene or cell therapy. * Participant is currently enrolled in a clinical trial or receiving an investigational therapy. * Participant has clinically significant underlying liver disease.