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.