Inclusion Criteria:  Participants must have confirmed diagnosis of infantile-onset Pompe disease defined as: the presence of 2 lysosomal acid α-glucosidase (GAA) pathogenic variants and a documented GAA deficiency from blood, skin, or muscle tissue; or the presence of 1 GAA pathogenic variant and a documented GAA deficiency from blood, skin and muscle tissue in 2 separate samples (from either 2 different tissues or from the same tissue but at 2 different sampling dates). Participants must have established cross-reactive immunological material (CRIM) status available prior to enrollment.  Participants must have cardiomyopathy at the time of diagnosis: ie, LVMI equivalent to mean age specific LVMI  1 standard deviation for participants diagnosed by newborn screening or sibling screening; 2 standard deviation for participants diagnosed by clinical evaluation. Parents or legally authorized representative(s) must be capable of giving signed informed consent.  Exclusion Criteria:  Participants with symptoms of respiratory insufficiency, including any ventilation use (invasive or noninvasive) at the time of enrollment. Participants with major congenital abnormality. Participants with clinically significant organic disease (with the exception of symptoms relating to Pompe disease). Participant received enzyme-replacement therapy (ERT) with recombinant human acid α glucosidase (rhGAA) from any source. Participant who has previously been treated in any clinical trial of avalglucosidase alfa. Participant not suitable for participation, whatever the reason, as judged by the Investigator, including medical or clinical conditions, or participants potentially at risk of noncompliance to study procedures.