The main objectives of the study are to evaluate safety and efficacy of repeated treatment with recombinant human alfa-mannosidase of patients with alfa-mannosidosis aged less than 6 years

The Primary endpoints of the study include:

• Safety and tolerability of velmanase alfa as per Adverse events (AEs, including IRR), vital signs, laboratory parameters (hematology, biochemistry and urinanalysis)
• Detection of anti-velmanase alfa antibodies and neutralizing/inhibitory antibodies

The Secondary endpoints include changes from baseline to 24 months for the following parameters. Efficacy outcomes:

• Serum oligosaccharides
• Functional capacity: Peabody Developmental Motor Scale - 2nd edition (PDMS-2) scores, Mullen's Scale of Early Learning (MSEL) scores, Bruininks-Oseretsky Test Of Motor Proficiency-2nd Edition (BOT-2), when applicable by age (from 4 years) or upon the judgment of the physician
• Endurance: 3-Minute Stair Climb Test (3MSCT) and 6-Minute Walk Test (6MWT) in pediatric patients from 4 years of age, or when applicable according to the judgment of the physician, 2-Minute Walk Test (2MWT) in pediatric patients below 4 years of age, or when applicable according to the judgment of the physician
• Hearing evaluation: Otoacoustic Emissions (OAE) testing, Automatic Auditory Brainstem Response (A-ABR) audiometry
• Immunological profile, when applicable upon the judgment of the physician:
• CSF biomarkers: Tau protein (Tau), Neurofilament Protein Light (NFL), Glial Fibrillary Acidic Protein (GFAp), Oligosaccharides
• Assessment of quality of life via Questionnaire to parents
• Assessment of mannose-rich oligosaccharides in brain tissue, MRI
• Pharmacokinetic parameters

Inclusion Criteria:

1. Patient's custodial parent(s) must provide signed ICF prior to the involvement of the patient in any trial-related activities
2. The subject's custodial parent(s) must have the ability to comply with the protocol
3. The subject must have a confirmed diagnosis of alpha-mannosidosis as defined by alpha-mannosidase activity in leukocytes or fibroblasts < 10% of normal activity (historical data)
4. The subject must have an age at the time of screening < 6 years.

Exclusion Criteria:

1. The subject's diagnosis cannot be confirmed by alpha-mannosidase activity < 10% of normal activity
2. Presence of known chromosomal abnormality and syndromes affecting psychomotor development, other than alpha-mannosidosis
3. History of BMT
4. Presence of known clinically significant cardiovascular, hepatic, pulmonary, or renal disease or other medical conditions that, in the opinion of the Investigator, would preclude participation in the trial
5. Any other medical condition or serious intercurrent illness, or extenuating circumstance that, in the opinion of the Investigator, would preclude participation in the trial
6. Planned major surgery that, in the opinion of the Investigator, would preclude participation in the trial
7. Participation in other interventional trials testing the IMP within the last 3 months.