The primary objective of this study is to assess the pharmacokinetics and pharmacodynamics of gefurulimab in pediatric participants with AChR+ gMG for the duration of the study.

Inclusion Criteria:

United States of America (USA) specific inclusion criterion:

• Participant must be 12 to < 18 years of age at the time of signing the informed consent/assent.
• All participants must be vaccinated against meningococcal infection from serogroups A, B, C, W, and Y within 3 years and at least 2 weeks prior to the first dose of study intervention administration.

Rest of World (ROW) specific inclusion criteria:

• Participant must be 6 to < 18 years of age at the time of signing the informed consent/assent.
• All participants must be vaccinated against meningococcal infection from serogroups A, C, W, Y (and B where available) within 3 years prior to study intervention on Day 1. If vaccination occurs < 2 weeks prior to Day 1, the participants will receive prophylactic antibiotics for at least 2 weeks after initial meningococcal vaccination for serogroups A,C,W,Y (and B, where available)

Global inclusion criteria:

• Diagnosis of MG with generalized muscle weakness meeting the clinical criteria defined by Myasthenia Gravis Foundation of America (MGFA) Class II, III or IV
• Positive serological test for autoantibodies against AChR

Exclusion Criteria:

• History of thymectomy, or any other thymic surgery within 12 months prior to Screening
• Untreated thymic malignancy, carcinoma, or thymoma
• History of Neisseria meningitidis infection
• Pregnancy, breastfeeding, or intention to conceive during the course of the study

Alexion has a public commitment to allow requests for access to study data and will be supplying a protocol, CSR, and plain language summaries.