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

In this study, researchers will learn about a study drug called BIIB115 in healthy adult male volunteers and in participants with spinal muscular atrophy (SMA). This study will focus on children with SMA. The main objective of the study is to learn about the safety of BIIB115 and how participants respond to different doses of BIIB115. The main question researchers want to answer is: • How many participants have adverse events and serious adverse events during the study? Adverse events are unwanted health problems that may or may not be caused by the study drug. Researchers will also learn about how the body processes BIIB115. They will do this by measuring the levels of BIIB115 in both the blood and the cerebrospinal fluid, also known as the CSF. This is the fluid around the brain and spinal cord. The study will be split into 2 parts - Part A and Part B. During Part A: * After screening, healthy volunteers will be randomly placed into 1 of 4 groups to receive either BIIB115 or a placebo. A placebo looks like the study drug but contains no real medicine. * Participants will receive a single dose of either BIIB115 or the placebo as an injection directly into the spinal canal on Day 1. * Neither the researchers nor the participants will know if the participants will receive BIIB115 or the placebo. * The Part A treatment and follow-up period will last for 13 months. * Participants will have up to 6 clinic visits and 4 phone calls. During Part B: * After screening, children with SMA will be placed into 1 of 2 groups to receive BIIB115. * The doses of each group will be decided based on the results of Part A. * Both researchers and participants will know they are receiving BIIB115. * Participants will first receive 2 total doses of BIIB115 given at 2 different times. * The Part B treatment and follow-up period will last for 24 months. * Participants will have up to 14 clinic visits and 6 phone calls. Part B Long-Term Extension: * After completing the 25 months