VAL-1221 Delivered Intravenously in Ambulatory and Ventilator-free Participants With Late-Onset Pompe Disease

Inclusion Criteria: * Participant is able and willing to provide informed consent prior to any study procedures are performed * Diagnosis of GSDII based on one of the following: * Endogenous cultured skin fibroblast GAA activity less than (\<) 40 percent (%) of adult normal level * Endogenous whole blood or dried blood spot GAA activity in deficiency range * Genetic analysis showing pathogenic variants in both alleles * Onset of Pompe disease-related symptoms after 1 year of age * Previously treated with Myozyme or Lumizyme for at least 12 months and on a stable regimen for the past 6 months * Sexually active participants who are willing to use an acceptable method of contraception (abstinence, oral contraceptives, barrier method with spermicide, surgical sterilization, implanted or injectable contraceptives with a stable dose for at least 1 month prior to Baseline, hormonal intra-uterine device \[IUD\] inserted at least 1 month prior to Baseline) during the study and for 30 days after completion of treatment * If participant is female and not considered to be of childbearing potential, she is at least 2 years post-menopause, has undergone a tubal ligation, a total hysterectomy or bilateral oophorectomy * If participant is female and of childbearing potential, she has a negative serum pregnancy test during screening and Baseline and must be willing to undergo pregnancy testing at specific intervals during the study * Participant meets at least one of the following criteria: greater than (\>) 30% and \<80% predicted upright forced volume capacity (FVC) or participant is able to walk \>20% but \<80% predicted normal on 6-minute walk test with or without use of assistive devices * Able to comply with protocol requirements Exclusion Criteria: * Cardiac involvement in first year of life * Anti-GAA antibody titers \>1:51,200 at two time points * Prior use of chaperone therapy for GSD-II within the last 12 months * Use of immunosuppressive medication other than glucocorticoids within 6 months prior to study enrollment * Use of invasive ventilatory assistance other than Bilevel Positive Airway Pressure (BiPAP) at night or during periods of rest * Has received any investigational medication or has enrolled in any study involving investigational drugs or therapies within 30 days prior to first dose of study drug * Start of or change in usual regimen of albuterol or respiratory muscle training within 30 days prior to first dose of study drug * History of sensitivity to any of the constituents of the study drug * Participant is breastfeeding or planning to become pregnant or to breastfeed during the study or is currently breastfeeding * Participant has a medical condition or circumstance that, in the opinion of the investigator, might compromise the participant's ability to comply with the protocol or the participant's well-being or safety * Participant has any condition that, in the view of the investigator, places the participant at high risk of poor treatment compliance or of not completing the study