A Study to Learn About the Safety of BIIB115 Injections and How BIIB115 is Processed in the Bodies of Healthy Adult Male Volunteers and of Pediatric Participants With Spinal Muscular Atrophy Who Previously Took Onasemnogene Abeparvovec

Key Inclusion Criteria:

Part A:

• Male healthy participants aged 18 to 55 years, inclusive

• Have a body mass index of 18 to 30 kilograms per meter square (kg/m^2), inclusive

• Must be in good health as determined by the investigator, based on medical historyand screening evaluations

Part B:

• Age 0.5 to 12 years old, inclusive, at the time of informed consent

• Weight ≥7 kg at the time of informed consent

• Genetic diagnosis of SMA (5q SMA homozygous survival motor neuron 1 (SMN1) genedeletion or mutation or compound heterozygous mutation)

• Survival motor neuron 2 (SMN2) copy number ≥1

• Must have received intravenous (IV) onasemnogene abeparvovec per the approved labelor per guidelines including the steroid regimen and monitoring specified therein

• Treatment with onasemnogene abeparvovec ≥180 days prior to first BIIB115 dose

• Potential for improvement due to suboptimal clinical status secondary to SMA, asdetermined by the Investigator

Part B LTE

• Completion of the assessments in Part B

• Meets age-appropriate institutional criteria for use of anesthesia/sedation, if useis planned for study procedures (as assessed by the Investigator and eitheranesthesiologist or pulmonologist).

Key Exclusion Criteria:

Part A:

• Any reason, anatomical or otherwise (including abnormal hematology/coagulation),that presents increase of risk of complication from multiple lumbar puncture (LP)procedures required for dosing and CSF collection, per the investigator discretion

• History of any clinically significant cardiac, endocrine, gastrointestinal,hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic,dermatologic, psychiatric, or renal disease, or other major disease, as determinedby the Investigator

• Chronic, recurrent, or serious infection, as determined by the investigator, within 90 days prior to screening or between screening and Day -1

• Current enrollment or a plan to enroll in any interventional clinical study of adrug, biologic, or device, in which an investigational treatment or approved therapyfor investigational use is administered within 3 months (or 5 half-lives of theagent, whichever is longer) prior to randomization

Part B:

• Severe or serious AEs related to onasemnogene abeparvovec therapy that are ongoingduring Screening

• Interval of <180 days between onasemnogene abeparvovec therapy and first BIIB115dose

• Ongoing steroid treatment following onasemnogene abeparvovec at time of screening

• History of drug induced liver injury or liver failure per Hy's law definition

• History of thrombotic micrangiopathy

• Treatment with any SMN2-splicing modifier (nusinersen or risdiplam) after receivingonasemnogene abeparvovec. Treatment with nusinersen <12 months from the first doseof BIIB115.

• Any reason, anatomical or otherwise (including abnormal hematology/coagulation),that presents increase of risk of complication from the LP procedures, CSFcirculation, or safety assessments, including a history of hydrocephalus orimplanted shunt for CSF drainage.

• Permanent ventilation, defined as tracheostomy or ≥16 hours ventilation /daycontinuously for >21 days in the absence of an acute reversible event

Part B LTE:

• Any new condition or worsening of an existing condition that, according to theInvestigator, would make the participant unsuitable for inclusion, could interferewith the assessment of safety, or would compromise the ability of the participant toundergo study procedures.

• Clinically significant abnormalities in hematology, blood chemistry parameters, orelectrocardiograms (ECGs) prior to first LTE visit that would make the participantunsuitable for inclusion as assessed by the Investigator.

• Treatment with an approved SMN2-splicing modifier (nusinersen or risdiplam).

NOTE: Other protocol-defined Inclusion/Exclusion criteria may apply.