Pre-Symptomatic Study of Intravenous Onasemnogene Abeparvovec-xioi in Spinal Muscular Atrophy (SMA) for Patients With Multiple Copies of SMN2

Inclusion Criteria: * Age ≤6 weeks (≤42 days) at time of dose * Ability to tolerate thin liquids as demonstrated through a formal bedside swallowing test * Compound muscle action potential (CMAP) ≥2mV at Baseline; centralized review of CMAP data will be conducted * Gestational age of 35 to 42 weeks * Parent(s)/legal guardian(s) willing and able to complete the informed consent process and comply with study procedures and visit schedule * Patients with pre-symptomatic SMA Type 1 as determined by the following features: a. 2 copies of SMN2 (n ≥14) * Patients with pre-symptomatic SMA Type 2 as determined by the following features: 1. 3 copies of SMN2 (n ≥12) Exclusion Criteria: * Weight at screening visit \<2 kg * Hypoxemia (oxygen saturation \<96% awake or asleep without any supplemental oxygen or respiratory support) at the screening visit or for altitudes \>1000 m, oxygen saturation \<92% awake or asleep without any supplemental oxygen or respiratory support at the screening visit * Any clinical signs or symptoms at screening or immediately prior to dosing that are, in the opinion of the Investigator, strongly suggestive of SMA * Tracheostomy or current prophylactic use or requirement of noninvasive ventilatory support at any time and for any duration prior to screening or during the screening period * Patients with signs of aspiration/inability to tolerate nonthickened liquids based on a formal swallowing test performed as part of screening or patients receiving any non-oral feeding method * Clinically significant abnormal laboratory values (gamma-glutamyl transferase \[GGT\], Alanine transaminase \[ALT\], and aspartate aminotransferase \[AST\], or total bilirubin \> 2 × the upper limit of normal \[ULN\], creatinine ≥ 1.0 mg/dL, hemoglobin \[Hgb\] \< 8 or \> 18 g/dL; white blood cell \[WBC\] \> 20,000 per cmm) prior to gene replacement therapy. Patients with an elevated bilirubin level that is unequivocally the result of neonatal jaundice shall not be excluded * Treatment with an investigational or commercial product, including nusinersen, given for the treatment of SMA. This includes any history of gene therapy, prior antisense oligonucleotide treatment, or cell transplantation. * Patients whose weight-for-age is below the third percentile based on World Health Organization (WHO) Child Growth Standards * Biological mother with active viral infection as determined by screening laboratory samples (includes human immunodeficiency virus \[HIV\] or positive serology for hepatitis B or C) • Biological mothers with clinical suspicion of Zika virus that meet Centers for Disease Control and Prevention (CDC) Zika virus epidemiological criteria including history of residence in or travel to a geographic region with active Zika transmission at the time of travel will be tested for Zika virus RNA. Positive results warrant confirmed negative Zika virus RNA testing in the patient prior to enrollment. * Serious nonrespiratory tract illness requiring systemic treatment and/or hospitalization within 2 Weeks prior to screening * Upper or lower respiratory infection requiring medical attention, medical intervention, or increase in supportive care of any manner within 4 Weeks prior to dosing * Severe nonpulmonary/respiratory tract infection within 4 Weeks before administration of gene replacement therapy or concomitant illness that, in the opinion of the Investigator or Sponsor medical monitor, creates unnecessary risks for gene replacement therapy such as: * Major renal or hepatic impairment * Known seizure disorder * Diabetes mellitus * Idiopathic hypocalciuria * Symptomatic cardiomyopathy * Known allergy or hypersensitivity to prednisolone or other glucocorticosteroids or their excipients * Previous, planned or expected major surgical procedure including scoliosis repair surgery/procedure during the study assessment period * Concomitant use of any of the following: drugs for treatment of myopathy or neuropathy, agents used to treat diabetes mellitus, or ongoing immunosuppressive therapy, plasmapheresis, immunomodulators such as adalimumab, immunosuppressive therapy within 4 Weeks prior to gene replacement therapy * AntiAAV9 antibody titer \>1:50 as determined by Enzyme-linked Immunosorbent Assay (ELISA) binding immunoassay • Should a potential patient demonstrate AntiAAV9 antibody titer \>1:50, he or she may receive retesting inside the 30-Day screening period and will be eligible to participate if the AntiAAV9 antibody titer upon retesting is ≤1:50, provided the \<6 Week age requirement at the time of dosing is still met * Biological mother involved with the care of the child refuses anti-AAV9 antibody testing prior to dosing