• Evidence of signed and dated informed consent document(s) indicating that the subject has been informed of all pertinent aspects of the trial. Subjects must be willing and able to comply with the protocol, complete study assessments, and return for follow-up visits.

• Male or female subjects ≥ 18 years old.

• Diagnosis of SN MG defined as: (a) clinical syndrome consistent with a diagnosis of MG, and not otherwise explained by another condition, (b) abnormal neuromuscular transmission test results demonstrated by single-fiber electromyography or repetitive nerve stimulation; and (c) negative serologic test for anti-AChR and anti- MuSK antibodies as confirmed at screening, (d) limited, if any, response to therapy with immunotherapy and/or antiacetylcholinesterase (AChE) treatment. Further testing for low affinity antibodies to rapsyn-clustered AChR by cell-based assays will be done at baseline and the results included as part of subgroup analysis. All patients will have a negative genetic test for congenital myasthenic syndromes by history or at baseline to exclude the possibility of congenital myasthenic syndrome mimicking SN MG.

• MGFA Clinical Classification Class II, III, or IV at the time of screening and baseline.

• Moderate to severe myasthenia gravis as defined by a generalized myasthenia gravis impairment index score \> 11 or MG-ADL score of at least 5 (with \>50% of the score due to non-ocular symptoms) and a PASS response of No and a SSQ of \< 70% with at least 6 months of historical data as the baseline.

• Stable or worsening MG as defined by MGII remaining stable or increasing in the 4 week run-in interval.

• Patients are required to be on a stable dose of their MG treatment (Standard of care-SoC) for at least one month prior to screening. The SoC is limited to AChE inhibitors, steroids and NSISTs (e.g., azathioprine, methotrexate, cyclosporine, tacrolimus, and mycophenolate mofetil. There is no requirement for specific generalised myasthenia gravis therapies.

• Patients who discontinued early from previous trials of efgartigimod for reasons other than pregnancy, rescue therapy or a SAE can be included.

• Females of childbearing potential who are sexually active with a non-sterilized male partner must be willing to use at least one highly effective contraception method from the time of screening and for 3 months after the final dose of efgartigimod.

• Non-sterilized males who are sexually active with a female partner of childbearing potential must be willing to use a condom for the duration of the study and for 3 months after the last dose of efgartigimod. Because male condom is not a highly effective contraception method, it is strongly recommended that female partners of a male study subject also use a highly effective method of contraception throughout this period.

• Vital signs, electrocardiogram (ECG), and laboratory parameters within the normal ranges at screening, or, if outside normal ranges, deemed not clinically significant by the Investigator.

• Patient has documented IgG \>6 g/L within one month of screening

• Vaccinated for COVID-19 at least 2 weeks prior to screening visit.